Sulfur-containing phosphor powders, methods for making phosphor powders and devices incorporating same

ABSTRACT

Sulfur-containing phosphor powders, methods for making phosphor powders and devices incorporating same. The powders have a small particle size, narrow particle size distribution and are substantially spherical. The method of the invention permits the continuous production of such powders. The invention also relates to products such as display devices incorporating such phosphor powders.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to phosphor powders, methodsfor producing phosphor powders and devices incorporating the powders. Inparticular, the present invention is directed to sulfur-containingphosphor powders having small average particle size, a narrow particlesize distribution, high crystallinity and a spherical morphology. Thepresent invention also relates to a method for continuously producingsuch sulfur containing phosphor powders and to devices that incorporatethe powders such as flat panel display devices.

[0003] 2. Description of Related Art

[0004] Phosphors are compounds that are capable of emitting usefulquantities of radiation in the visible and/or ultraviolet spectrums uponexcitation of the phosphor compound by an external energy source. Due tothis property, phosphor compounds have long been utilized in cathode raytube (CRT) display devices, such as televisions, computers and similardevices. Typically, inorganic phosphor compounds include a host materialdoped with a small amount of an activator ion.

[0005] More recently, phosphor compounds, including phosphors inparticulate form, have been utilized in many advanced display devicesthat provide illuminated text, graphics or video output. In particular,there has been significant growth in the field of flat panel displaydevices such as liquid crystal displays plasma displays, thick film andthin film electroluminescent displays and field emission displays.

[0006] Liquid crystal displays (LCD) use a low power electric field tomodify a light path and are commonly used in wristwatches, pockettelevisions, gas pumps, pagers and similar devices. Active matrix liquidcrystal displays (AMLCD) are commonly used for laptop computers. Plasmadisplay panels (PDP) utilize a gas, trapped between transparent layers,that emits ultraviolet light when excited by an electric field. Theultraviolet light stimulates phosphors on the screen to emit visiblelight. Plasma displays are particularly useful for larger displays, suchas greater than about 20 diagonal inches. Thin film and thick filmelectroluminescent displays (TFEL) utilize a film of phosphorescentmaterial trapped between glass plates and electrodes to emit light in anelectric field. Such displays are typically used in commercialtransportation vehicles, factory floors and emergency rooms. Fieldemission displays (FED) are similar in principle to CRT's, whereinelectrons emitted from a tip excite phosphors, which then emit light ofa preselected color. Phosphor powders are also utilized inelectroluminescent lamps (EL), which include phosphor powder depositedon a polymer substrate which emits light when an electric field isapplied.

[0007] There are a number of requirements for phosphor powders, whichcan vary dependent upon the specific application of the powder.Generally, phosphor powders should have one or more of the followingproperties: high purity; high crystallinity; small particle size; narrowparticle size distribution; spherical morphology; controlled surfacechemistry; homogenous distribution of the activator ion; gooddispersibility, and low porosity. The proper combination of theforegoing properties will result in a phosphor powder with highluminescent intensity and long lifetime that can be used in manyapplications. It is also advantageous for many applications to providephosphor powders that are surface passivated or coated, such as with athin, uniform dielectric or semiconducting coating.

[0008] Numerous methods have been proposed for producingsulfur-containing phosphor particles. One such method is referred to asthe solid-state method. In this process, solid phosphor precursormaterials are mixed and are heated so that the precursors react in thesolid-state and form a powder of the phosphor material. It is difficultto produce a uniform and homogenous phosphor powder by solid statemethods. Further, solid-state routes, and many other production methods,utilize a grinding step to reduce the particle size of the powders.Mechanical grinding damages the surface of the phosphor, forming deadlayers which inhibit the brightness of the phosphor powders.

[0009] Phosphor powders have also been made by liquid precipitationmethods. In these methods, a solution which includes phosphor particleprecursors is chemically treated to precipitate phosphor particles orphosphor particle precursors. The precipitated compounds are typicallycalcined at an elevated temperature to produce the final phosphormaterial. An example of this type of preparation is disclosed in U.S.Pat. No. 5,413,736 by Nishisu et al. In yet another method, phosphorparticle precursors or phosphor particles are dispersed in a solutionwhich is then spray dried to evaporate the liquid. The phosphorparticles are thereafter sintered in the solid state at an elevatedtemperature to crystallize the powder and form the phosphor compound.Such a process is exemplified by U.S. Pat. No. 4,948,527 by Ritsko etal. and U.S. Pat. No. 3,709,826 by Pitt et al.

[0010] International Application No. PCT/US95/07869 by Kane discloses aprocess for preparing oxysulfide phosphor particles having a particlesize of 1 μm or less that are spherical in shape. In this process,hydroxycarbonate compounds are precipitated from solution. Thehydroxycarbonates are then heated in oxygen to form an oxide which isthen heated in a sulfur-containing flux to form the oxysulfide phosphor.

[0011] U.S. Pat. No. 3,676,358 discloses a process wherein a solution ofprecursor nitrates are atomized and heated at 400° F. to dry theparticles. The particles are then passed through a flame to react andform the phosphor.

[0012] Tohge et al. in an article entitled “Formation of Fine Particlesof Zinc Sulfide from Thiourea Complexes by Spray Pyrolysis” JapaneseJournal of Applied Physics, Vol. 34, 1995, pgs. 207-209) discloseparticles of ZnS fabricated by ultrasonic spray pyrolysis of an aqueoussolution. The particles are spherical with a smooth surface and have asize range of from 0.5 to 1.3 μm. It is disclosed that particles reactedat 400° C. are amorphous whereas particles reacted at 600° C. and highershow crystalline phases. Partial oxidation of the zinc sulfide above900° C. was also observed. Tohge et al. have also disclosed theformation of cadmium sulfide by a similar process in an article entitled“Formation of CdS fine particles by spray-pyrolysis” (Journal MaterialScience Letter, Vol. 14, 1995, pgs. 1388-1390).

[0013] Despite the foregoing, there remains a need for phosphor powderbatches that include particles having a small size, substantiallyspherical morphology, narrow particle size distribution, a high degreeof crystallinity and good homogeneity, which result in high luminescentintensity. The powder should have good dispersibility and the ability tobe fabricated into thin layers having uniform thickness, resulting in adevice with high brightness.

SUMMARY OF THE INVENTION

[0014] The present invention is directed to sulfur-containing phosphorpowder batches preferably having a small particle size, narrow particlesize distribution, spherical morphology and high crystallinity. Thepresent invention also provides methods for producing suchsulfur-containing phosphor powder batches and devices incorporating thepowders.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a process block diagram showing one embodiment of theprocess of the present invention.

[0016]FIG. 2 is a side view of a furnace and showing one embodiment ofthe present invention for sealing the end of a furnace tube.

[0017]FIG. 3 is a view of the side of an end cap that faces away fromthe furnace shown in FIG. 2.

[0018]FIG. 4 is a view of the side of an end cap that faces toward thefurnace shown in FIG. 2.

[0019]FIG. 5 is a side view in cross section of one embodiment ofaerosol generator of the present invention.

[0020]FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

[0021]FIG. 7 is a top view of a transducer mounting plate for a 400transducer array for use in an ultrasonic generator of the presentinvention.

[0022]FIG. 8 is a side view of the transducer mounting plate shown inFIG. 7.

[0023]FIG. 9 is a partial side view showing the profile of a singletransducer mounting receptacle of the transducer mounting plate shown inFIG. 7.

[0024]FIG. 10 is a partial side view in cross-section showing analternative embodiment for mounting an ultrasonic transducer.

[0025]FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

[0026]FIG. 12 is a top view of a liquid feed box having a bottomretaining plate to assist in retaining a separator for use in an aerosolgenerator of the present invention.

[0027]FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

[0028]FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

[0029]FIG. 15 shows a partial top view of gas tubes positioned in aliquid feed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

[0030]FIG. 16 shows one embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

[0031]FIG. 17 shows another embodiment for a gas distributionconfiguration for the aerosol generator of the present invention.

[0032]FIG. 18 is a top view of one embodiment of a gas distributionplate/gas tube assembly of the aerosol generator of the presentinvention.

[0033]FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

[0034]FIG. 20 shows one embodiment for orienting a transducer in theaerosol generator of the present invention.

[0035]FIG. 21 is a top view of a gas manifold for distributing gaswithin an aerosol generator of the present invention.

[0036]FIG. 22 is a side view of the gas manifold shown in FIG. 21.

[0037]FIG. 23 is a top view of a generator lid of a hood design for usein an aerosol generator of the present invention.

[0038]FIG. 24 is a side view of the generator lid shown in FIG. 23.

[0039]FIG. 25 is a process block diagram of one embodiment in thepresent invention including an aerosol concentrator.

[0040]FIG. 26 is a top view in cross section of a virtual impactor thatmay be used for concentrating an aerosol according to the presentinvention.

[0041]FIG. 27 is a front view of an upstream plate assembly of thevirtual impactor shown in FIG. 26.

[0042]FIG. 28 is a top view of the upstream plate assembly shown in FIG.27.

[0043]FIG. 29 is a side view of the upstream plate assembly shown inFIG. 27.

[0044]FIG. 30 is a front view of a downstream plate assembly of thevirtual impactor shown in FIG. 26.

[0045]FIG. 31 is a top view of the downstream plate assembly shown inFIG. 30.

[0046]FIG. 32 is a side view of the downstream plate assembly shown inFIG. 30.

[0047]FIG. 33 is a process block diagram of one embodiment of theprocess of the present invention including a droplet classifier.

[0048]FIG. 34 is a top view in cross section of an impactor of thepresent invention for use in classifying an aerosol.

[0049]FIG. 35 is a front view of a flow control plate of the impactorshown in FIG. 34.

[0050]FIG. 36 is a front view of a mounting plate of the impactor shownin FIG. 34.

[0051]FIG. 37 is a front view of an impactor plate assembly of theimpactor shown in FIG. 34.

[0052]FIG. 38 is a side view of the impactor plate assembly shown inFIG. 37.

[0053]FIG. 39 shows a side view in cross section of a virtual impactorin combination with an impactor of the present invention forconcentrating and classifying droplets in an aerosol.

[0054]FIG. 40 is a process block diagram of one embodiment of thepresent invention including a particle cooler.

[0055]FIG. 41 is a top view of a gas quench cooler of the presentinvention.

[0056]FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

[0057]FIG. 43 is a side view of a perforated conduit of the quenchcooler shown in FIG. 41.

[0058]FIG. 44 is a side view showing one embodiment of a gas quenchcooler of the present invention connected with a cyclone.

[0059]FIG. 45 is a process block diagram of one embodiment of thepresent invention including a particle coater.

[0060]FIG. 46 is a block diagram of one embodiment of the presentinvention including a particle modifier.

[0061]FIG. 47 shows cross sections of various particle morphologies ofsome composite particles manufacturable according to the presentinvention.

[0062]FIG. 48 shows a side view of one embodiment of apparatus of thepresent invention including an aerosol generator, an aerosolconcentrator, a droplet classifier, a furnace, a particle cooler, and aparticle collector.

[0063]FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

[0064]FIG. 50 illustrates a schematic view of a CRT device according toan embodiment of the present invention.

[0065]FIG. 51 illustrates a schematic representation of pixels on aviewing screen of a CRT device according to an embodiment of the presentinvention.

[0066]FIG. 52 schematically illustrates a plasma display panel accordingto an embodiment of the present invention.

[0067]FIG. 53 schematically illustrates a field emission displayaccording to an embodiment of the present invention.

[0068]FIG. 54 illustrates pixel regions on a display device according tothe prior art.

[0069]FIG. 55 illustrates pixel regions on a display device according toan embodiment of the present invention.

[0070]FIG. 56 schematically illustrates a cross-section of anelectroluminescent display device according to an embodiment of thepresent invention.

[0071]FIG. 57 schematically illustrates an exploded view of anelectroluminescent display device according to an embodiment of thepresent invention.

[0072]FIG. 58 illustrates an electroluminescent lamp according to anembodiment of the present invention.

[0073]FIG. 59 illustrates a photomicrograph of a sulfur-containingphosphor powder according to an embodiment of the present invention.

[0074]FIG. 60 illustrates a photomicrograph of a sulfur-containingphosphor powder according to an embodiment of the present invention.

[0075]FIG. 61 illustrates a photomicrograph of a sulfur-containingphosphor powder according to an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

[0076] The present invention is generally directed to sulfur-containingphosphor powders and methods for producing the powders, as well asdevices which incorporate the powders. As used herein, sulfur-containingphosphor powders, particles and compounds are those which incorporatethe host material which is a metal sulfide, oxysulfide or thiogallate.Specific examples of such sulfur-containing phosphor compounds aredetailed hereinbelow.

[0077] In one aspect, the present invention provides a method forpreparing a particulate product. A feed of liquid-containing, flowablemedium, including at least one precursor for the desired particulateproduct, is converted to aerosol form, with droplets of the medium beingdispersed in and suspended by a carrier gas. Liquid from the droplets inthe aerosol is then removed to permit formation in a dispersed state ofthe desired particles. Typically, the feed precursor is pyrolyzed in afurnace to make the particles. In one embodiment, the particles aresubjected, while still in a dispersed state, to compositional orstructural modification, if desired. Compositional modification mayinclude, for example, coating the particles. Structural modification mayinclude, for example, crystallization, recrystallization ormorphological alteration of the particles. The term powder is often usedherein to refer to the particulate product of the present invention. Theuse of the term powder does not indicate, however, that the particulateproduct must be dry or in any particular environment. Although theparticulate product is typically manufactured in a dry state, theparticulate product may, after manufacture, be placed in a wetenvironment, such as in a paste or slurry.

[0078] The process of the present invention is particularly well suitedfor the production of particulate products of finely divided particleshaving a small weight average size. In addition to making particleswithin a desired range of weight average particle size, with the presentinvention the particles may be produced with a desirably narrow sizedistribution, thereby providing size uniformity that is desired for manyapplications.

[0079] In addition to control over particle size and size distribution,the method of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

[0080] Referring now to FIG. 1, one embodiment of the process of thepresent invention is described. A liquid feed 102, including at leastone precursor for the desired particles, and a carrier gas 104 are fedto an aerosol generator 106 where an aerosol 108 is produced. Theaerosol 108 is then fed to a furnace 110 where liquid in the aerosol 108is removed to produce particles 112 that are dispersed in and suspendedby gas exiting the furnace 110. The particles 112 are then collected ina particle collector 114 to produce a particulate product 116.

[0081] As used herein, the liquid feed 102 is a feed that includes oneor more flowable liquids as the major constituent(s), such that the feedis a flowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 μm in size,preferably smaller than about 0.5 μm in size, and more preferablysmaller than about 0.3 μm in size and most preferably smaller than about0.1 μm in size. Most preferably, the suspended particles should be ableto form a colloid. The suspended particles could be finely dividedparticles, or could be agglomerate masses comprised of agglomeratedsmaller primary particles. For example, 0.5 μm particles could beagglomerates of nanometer-sized primary particles. When the liquid feed102 includes suspended particles, the particles typically comprise nogreater than about 25 to 50 weight percent of the liquid feed.

[0082] As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing chemical reaction.This could be the case, for example, when the liquid feed 102 includes,as a precursor material, suspended particles that are not chemicallymodified in the furnace 110. In any event, the particles 112 comprise atleast one component originally contributed by the precursor.

[0083] The liquid feed 102 may include multiple precursor materials,which may be present together in a single phase or separately inmultiple phases. For example, the liquid feed 102 may include multipleprecursors in solution in a single liquid vehicle. Alternatively, oneprecursor material could be in a solid particulate phase and a secondprecursor material could be in a liquid phase. Also, one precursormaterial could be in one liquid phase and a second precursor materialcould be in a second liquid phase, such as could be the case when theliquid feed 102 comprises an emulsion. Different components contributedby different precursors may be present in the particles together in asingle material phase, or the different components may be present indifferent material phases when the particles 112 are composites ofmultiple phases. Specific examples of preferred precursor materials arediscussed more fully below.

[0084] The carrier gas 104 may comprise any gaseous medium in whichdroplets produced from the liquid feed 102 may be dispersed in aerosolform. Also, the carrier gas 104 may be inert, in that the carrier gas104 does not participate in formation of the particles 112.Alternatively, the carrier gas may have one or more active component(s)that contribute to formation of the particles 112. In that regard, thecarrier gas may include one or more reactive components that react inthe furnace 110 to contribute to formation of the particles 112.Preferred carrier gas compositions are discussed more fully below.

[0085] The aerosol generator 106 atomizes the liquid feed 102 to formdroplets in a manner to permit the carrier gas 104 to sweep the dropletsaway to form the aerosol 108. The droplets comprise liquid from theliquid feed 102. The droplets may, however, also include nonliquidmaterial, such as one or more small particles held in the droplet by theliquid. For example, when the particles 112 are composite, ormulti-phase, particles, one phase of the composite may be provided inthe liquid feed 102 in the form of suspended precursor particles and asecond phase of the composite may be produced in the furnace 110 fromone or more precursors in the liquid phase of the liquid feed 102.Furthermore the precursor particles could be included in the liquid feed102, and therefore also in droplets of the aerosol 108, for the purposeonly of dispersing the particles for subsequent compositional orstructural modification during or after processing in the furnace 110.

[0086] An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

[0087] The aerosol generator 106 is capable of producing the aerosol 108such that it includes droplets having a weight average size in a rangehaving a lower limit of about 1 μm and preferably about 2 μm; and anupper limit of about 10 μm; preferably about 7 μm, more preferably about5 μm and most preferably about 4 μm. A weight average droplet size in arange of from about 2 μm to about 4 μm is more preferred for mostapplications, with a weight average droplet size of about 3 μm beingparticularly preferred for some applications. The aerosol generator isalso capable of producing the aerosol 108 such that it includes dropletsin a narrow size distribution. Preferably, the droplets in the aerosolare such that at least about 70 percent (more preferably at least about80 weight percent and most preferably at least about 85 weight percent)of the droplets are smaller than about 10 μm and more preferably atleast about 70 weight percent (more preferably at least about 80 weightpercent and most preferably at least about 85 weight percent) aresmaller than about 5 μm. Furthermore, preferably no greater than about30 weight percent, more preferably no greater than about 25 weightpercent and most preferably no greater than about 20 weight percent, ofthe droplets in the aerosol 108 are larger than about twice the weightaverage droplet size.

[0088] Another important aspect of the present invention is that theaerosol 108 may be generated without consuming excessive amounts of thecarrier gas 104. The aerosol generator 106 is capable of producing theaerosol 108 such that it has a high loading, or high concentration, ofthe liquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

[0089] This capability of the aerosol generator 106 to produce a heavilyloaded aerosol 108 is even more surprising given the high droplet outputrate of which the aerosol generator 106 is capable, as discussed morefully below. It will be appreciated that the concentration of liquidfeed 102 in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.2 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

[0090] The furnace 110 may be any suitable device for heating theaerosol 108 to evaporate liquid from the droplets of the aerosol 108 andthereby permit formation of the particles 112. The maximum averagestream temperature, or reaction temperature, refers to the maximumaverage temperature that an aerosol stream attains while flowing throughthe furnace. This is typically determined by a temperature probeinserted into the furnace. Preferred reaction temperatures according tothe present invention are discussed more fully below.

[0091] Although longer residence times are possible, for manyapplications, residence time in the heating zone of the furnace 110 ofshorter than about 4 seconds is typical, with shorter than about 2seconds being preferred, shorter than about 1 second being morepreferred, shorter than about 0.5 second being even more preferred, andshorter than about 0.2 second being most preferred. The residence timeshould be long enough, however, to assure that the particles 112 attainthe desired maximum stream temperature for a given heat transfer rate.In that regard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximum streamtemperature not be attained in the furnace 110 until substantially atthe end of the heating zone in the furnace 110. For example, the heatingzone will often include a plurality of heating sections that are eachindependently controllable. The maximum stream temperature shouldtypically not be attained until the final heating section, and morepreferably until substantially at the end of the last heating section.This is important to reduce the potential for thermophoretic losses ofmaterial. Also, it is noted that as used herein, residence time refersto the actual time for a material to pass through the relevant processequipment. In the case of the furnace, this includes the effect ofincreasing velocity with gas expansion due to heating.

[0092] Typically, the furnace 110 will be a tube-shaped furnace, so thatthe aerosol 108 moving into and through the furnace does not encountersharp edges on which droplets could collect. Loss of droplets tocollection at sharp surfaces results in a lower yield of particles 112.More important, however, the accumulation of liquid at sharp edges canresult in re-release of undesirably large droplets back into the aerosol108, which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

[0093] The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

[0094] When a metallic tube is used in the furnace 110, it is preferablya high nickel content stainless steel alloy, such as a 330 stainlesssteel, or a nickel-based super alloy. As noted, one of the majoradvantages of using a metallic tube is that the tube is relatively easyto seal with other process equipment. In that regard, flange fittingsmay be welded directly to the tube for connecting with other processequipment. Metallic tubes are generally preferred for making particlesthat do not require a maximum tube wall temperature of higher than about1100° C. during particle manufacture.

[0095] When higher temperatures are required, ceramic tubes aretypically used. One major problem with ceramic tubes, however, is thatthe tubes can be difficult to seal with other process equipment,especially when the ends of the tubes are maintained at relatively hightemperatures, as is often the case with the present invention.

[0096] One configuration for sealing a ceramic tube is shown in FIGS. 2,3 and 4. The furnace 110 includes a ceramic tube 374 having an end cap376 fitted to each end of the tube 374, with a gasket 378 disposedbetween corresponding ends of the ceramic tube 374 and the end caps 376.The gasket may be of any suitable material for sealing at thetemperature encountered at the ends of the tubes 374. Examples of gasketmaterials for sealing at temperatures below about 250° C. includesilicone, TEFLON™ and VITON™. Examples of gasket materials for highertemperatures include graphite, ceramic paper, thin sheet metal, andcombinations thereof.

[0097] Tension rods 380 extend over the length of the furnace 110 andthrough rod holes 382 through the end caps 376. The tension rods 380 areheld in tension by the force of springs 384 bearing against bearingplates 386 and the end caps 376. The tube 374 is, therefore, incompression due to the force of the springs 384. The springs 384 may becompressed any desired amount to form a seal between the end caps 376and the ceramic tube 374 through the gasket 378. As will be appreciated,by using the springs 384, the tube 374 is free to move to some degree asit expands upon heating and contracts upon cooling. To form the sealbetween the end caps 376 and the ceramic tube 374, one of the gaskets378 is seated in a gasket seat 388 on the side of each end cap 376facing the tube 374. A mating face 390 on the side of each of the endcaps 376 faces away from the tube 374, for mating with a flange surfacefor connection with an adjacent piece of equipment.

[0098] Also, although the present invention is described with primaryreference to a furnace reactor, which is preferred, it should berecognized that, except as noted, any other thermal reactor, including aflame reactor or a plasma reactor, could be used instead. A furnacereactor is, however, preferred, because of the generally even heatingcharacteristic of a furnace for attaining a uniform stream temperature.

[0099] The particle collector 114, may be any suitable apparatus forcollecting particles 112 to produce the particulate product 116. Onepreferred embodiment of the particle collector 114 uses one or morefilter to separate the particles 112 from gas. Such a filter may be ofany type, including a bag filter. Another preferred embodiment of theparticle collector uses one or more cyclone to separate the particles112. Other apparatus that may be used in the particle collector 114includes an electrostatic precipitator. Also, collection should normallyoccur at a temperature above the condensation temperature of the gasstream in which the particles 112 are suspended. Also, collection shouldnormally be at a temperature that is low enough to prevent significantagglomeration of the particles 112.

[0100] Of significant importance to the operation of the process of thepresent invention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

[0101] With continued reference to FIG. 5, a separator 126, in spacedrelation to the transducer discs 120, is retained between a bottomretaining plate 128 and a top retaining plate 130. Gas delivery tubes132 are connected to gas distribution manifolds 134, which have gasdelivery ports 136. The gas distribution manifolds 134 are housed withina generator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

[0102] During operation of the aerosol generator 106, as shown in FIG.5, the transducer discs 120 are activated by the transducer driver 144via the electrical cables 146. The transducers preferably vibrate at afrequency of from about 1 MHz to about 5 MHz, more preferably from about1.5 MHz to about 3 MHz. Frequently used frequencies are at about 1.6 MHzand about 2.4 MHz. Furthermore, all of the transducer discs 110 shouldbe operating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

[0103] Liquid feed 102 enters through a feed inlet 148 and flows throughflow channels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

[0104] The ultrasonic signals from the ultrasonic transducer discs 120cause atomization cones 162 to develop in the liquid feed 102 atlocations corresponding with the transducer discs 120. Carrier gas 104is introduced into the gas delivery tubes 132 and delivered to thevicinity of the atomization cones 162 via gas delivery ports 136. Jetsof carrier gas exit the gas delivery ports 136 in a direction so as toimpinge on the atomization cones 162, thereby sweeping away atomizeddroplets of the liquid feed 102 that are being generated from theatomization cones 162 and creating the aerosol 108, which exits theaerosol generator 106 through an aerosol exit opening 164.

[0105] Efficient use of the carrier gas 104 is an important aspect ofthe aerosol generator 106. The embodiment of the aerosol generator 106shown in FIG. 5 includes two gas exit ports per atomization cone 162,with the gas ports being positioned above the liquid medium 102 overtroughs that develop between the atomization cones 162, such that theexiting carrier gas 104 is horizontally directed at the surface of theatomization cones 162, thereby efficiently distributing the carrier gas104 to critical portions of the liquid feed 102 for effective andefficient sweeping away of droplets as they form about theultrasonically energized atomization cones 162. Furthermore, it ispreferred that at least a portion of the opening of each of the gasdelivery ports 136, through which the carrier gas exits the gas deliverytubes, should be located below the top of the atomization cones 162 atwhich the carrier gas 104 is directed. This relative placement of thegas delivery ports 136 is very important to efficient use of carrier gas104. Orientation of the gas delivery ports 136 is also important.Preferably, the gas delivery ports 136 are positioned to horizontallydirect jets of the carrier gas 104 at the atomization cones 162. Theaerosol generator 106 permits generation of the aerosol 108 with heavyloading with droplets of the carrier liquid 102, unlike aerosolgenerator designs that do not efficiently focus gas delivery to thelocations of droplet formation.

[0106] Another important feature of the aerosol generator 106, as shownin FIG. 5, is the use of the separator 126, which protects thetransducer discs 120 from direct contact with the liquid feed 102, whichis often highly corrosive. The height of the separator 126 above the topof the transducer discs 120 should normally be kept as small aspossible, and is often in the range of from about 1 centimeter to about2 centimeters. The top of the liquid feed 102 in the flow channels abovethe tops of the ultrasonic transducer discs 120 is typically in a rangeof from about 2 centimeters to about 5 centimeters, whether or not theaerosol generator includes the separator 126, with a distance of about 3to 4 centimeters being preferred. Although the aerosol generator 106could be made without the separator 126, in which case the liquid feed102 would be in direct contact with the transducer discs 120, the highlycorrosive nature of the liquid feed 102 can often cause prematurefailure of the transducer discs 120. The use of the separator 126, incombination with use of the ultrasonically transmissive fluid in thewater bath volume 156 to provide ultrasonic coupling, significantlyextending the life of the ultrasonic transducers 120. One disadvantageof using the separator 126, however, is that the rate of dropletproduction from the atomization cones 162 is reduced, often by a factorof two or more, relative to designs in which the liquid feed 102 is indirect contact with the ultrasonic transducer discs 102. Even with theseparator 126, however, the aerosol generator 106 used with the presentinvention is capable of producing a high quality aerosol with heavydroplet loading, as previously discussed. Suitable materials for theseparator 126 include, for example, polyamides (such as Kapton®membranes from DuPont) and other polymer materials, glass, andplexiglass. The main requirements for the separator 126 are that it beultrasonically transmissive, corrosion resistant and impermeable.

[0107] One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

[0108] One surprising finding with operation of the aerosol generator106 of the present invention is that the droplet loading in the aerosolmay be affected by the temperature of the liquid feed 102. It has beenfound that when the liquid feed 102 includes an aqueous liquid at anelevated temperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

[0109] The design for the aerosol generator 106 based on an array ofultrasonic transducers is versatile and is easily modified toaccommodate different generator sizes for different specialtyapplications. The aerosol generator 106 may be designed to include aplurality of ultrasonic transducers in any convenient number. Even forsmaller scale production, however, the aerosol generator 106 preferablyhas at least nine ultrasonic transducers, more preferably at least 16ultrasonic transducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

[0110]FIGS. 7-24 show component designs for an aerosol generator 106including an array of 400 ultrasonic transducers. Referring first toFIGS. 7 and 8, the transducer mounting plate 124 is shown with a designto accommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

[0111] As shown in FIGS. 7 and 8, four hundred transducer mountingreceptacles 174 are provided in the transducer mounting plate 124 formounting ultrasonic transducers for the desired array. With reference toFIG. 9, the profile of an individual transducer mounting receptacle 174is shown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

[0112] A preferred transducer mounting configuration, however, is shownin FIG. 10 for another configuration for the transducer mounting plate124. As seen in FIG. 10, an ultrasonic transducer disc 120 is mounted tothe transducer mounting plate 124 by use of a compression screw 177threaded into a threaded, receptacle 179. The compression screw 177bears against the ultrasonic transducer disc 120, causing an o-ring 181,situated in an o-ring seat 182 on the transducer mounting plate, to becompressed to form a seal between the transducer mounting plate 124 andthe ultrasonic transducer disc 120. This type of transducer mounting isparticularly preferred when the ultrasonic transducer disc 120 includesa protective surface coating, as discussed previously, because the sealof the o-ring to the ultrasonic transducer disc 120 will be inside ofthe outer edge of the protective seal, thereby preventing liquid frompenetrating under the protective surface coating from the edges of theultrasonic transducer disc 120.

[0113] Referring now to FIG. 11, the bottom retaining plate 128 for a400 transducer array is shown having a design for mating with thetransducer mounting plate 124 (shown in FIGS. 7-8). The bottom retainingplate 128 has eighty openings 184, arranged in four subgroups 186 oftwenty openings 184 each. Each of the openings 184 corresponds with fiveof the transducer mounting receptacles 174 (shown in FIGS. 7 and 8) whenthe bottom retaining plate 128 is mated with the transducer mountingplate 124 to create a volume for a water bath between the transducermounting plate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

[0114] Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

[0115] The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

[0116] Referring now to FIGS. 12-14, a plurality of gas tubefeed-through holes 202 extend through the vertically extending walls 192to either side of the assembly including the feed inlet 148 and feedoutlet 152 of the liquid feed box 190. The gas tube feed-through holes202 are designed to permit insertion therethrough of gas tubes 208 of adesign as shown in FIG. 14. When the aerosol generator 106 is assembled,a gas tube 208 is inserted through each of the gas tube feed-throughholes 202 so that gas delivery ports 136 in the gas tube 208 will beproperly positioned and aligned adjacent the openings 194 in the topretaining plate 130 for delivery of gas to atomization cones thatdevelop in the liquid feed box 190 during operation of the aerosolgenerator 106. The gas delivery ports 136 are typically holes having adiameter of from about 1.5 millimeters to about 3.5 millimeters.

[0117] Referring now to FIG. 15, a partial view of the liquid feed box190 is shown with gas tubes 208A, 208B and 208C positioned adjacent tothe openings 194 through the top retaining plate 130. Also shown in FIG.15 are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

[0118] Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

[0119] An alternative, and preferred, flow for carrier gas 104 is shownin FIG. 17. As shown in FIG. 17, carrier gas 104 is delivered from onlyone side of each of the gas tubes 208. This results in a sweep ofcarrier gas from all of the gas tubes 208 toward a central area 212.This results in a more uniform flow pattern for aerosol generation thatmay significantly enhance the efficiency with which the carrier gas 104is used to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

[0120] Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

[0121] Aerosol generation may also be enhanced through mounting ofultrasonic transducers at a slight angle and directing the carrier gasat resulting atomization cones such that the atomization cones aretilting in the same direction as the direction of flow of carrier gas.Referring to FIG. 20, an ultrasonic transducer disc 120 is shown. Theultrasonic transducer disc 120 is tilted at a tilt angle 114 (typicallyless than 10 degrees), so that the atomization cone 162 will also have atilt. It is preferred that the direction of flow of the carrier gas 104directed at the atomization cone 162 is in the same direction as thetilt of the atomization cone 162.

[0122] Referring now to FIGS. 21 and 22; a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

[0123] Referring now to FIGS. 23 and 24, the generator lid 140 is shownfor a 400 transducer array design. The generator lid 140 mates with andcovers the liquid feed box 190 (shown in FIGS. 12 and 13). The generatorlid 140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

[0124] Although the aerosol generator 106 produces a high qualityaerosol 108 having a high droplet loading, it is often desirable tofurther concentrate the aerosol 108 prior to introduction into thefurnace 110. Referring now to FIG. 25, a process flow diagram is shownfor one embodiment of the present invention involving such concentrationof the aerosol 108. As shown in FIG. 25, the aerosol 108 from theaerosol generator 106 is sent to an aerosol concentrator 236 whereexcess carrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

[0125] The aerosol concentrator 236 typically includes one or morevirtual impactors capable of concentrating droplets in the aerosol 108by a factor of greater than about 2, preferably by a factor of greaterthan about 5, and more preferably by a factor of greater than about 10,to produce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid feed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

[0126] Having a high droplet loading in aerosol feed to the furnaceprovides the important advantage of reducing the heating demand on thefurnace 110 and the size of flow conduits required through the furnace.Also, other advantages of having a dense aerosol include a reduction inthe demands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

[0127] The excess carrier gas 238 that is removed in the aerosolconcentrator 236 typically includes extremely small droplets that arealso removed from the aerosol 108. Preferably, the droplets removed withthe excess carrier gas 238 have a weight average size of smaller thanabout 1.5 μm, and more preferably smaller than about 1 μm and thedroplets retained in the concentrated aerosol 240 have an averagedroplet size of larger than about 2 μm. For example, a virtual impactorsized to treat an aerosol stream having a weight average droplet size ofabout three μm might be designed to remove with the excess carrier gas238 most droplets smaller than about 1.5 μm in size. Other designs arealso possible. When using the aerosol generator 106 with the presentinvention, however, the loss of these very small droplets in the aerosolconcentrator 236 will typically constitute no more than about 10 percentby weight, and more preferably no more than about 5 percent by weight,of the droplets originally in the aerosol stream that is fed to theconcentrator 236. Although the aerosol concentrator 236 is useful insome situations, it is normally not required with the process of thepresent invention, because the aerosol generator 106 is capable, in mostcircumstances, of generating an aerosol stream that is sufficientlydense. So long as the aerosol stream coming out of the aerosol generator102 is sufficiently dense, it is preferred that the aerosol concentratornot be used. It is a significant advantage of the present invention thatthe aerosol generator 106 normally generates such a dense aerosol streamthat the aerosol concentrator 236 is not needed. Therefore, thecomplexity of operation of the aerosol concentrator 236 and accompanyingliquid losses may typically be avoided.

[0128] It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

[0129] One embodiment of a virtual impactor that could be used as theaerosol concentrator 236 will now be described with reference to FIGS.26-32. A virtual impactor 246 includes an upstream plate assembly 248(details shown in FIGS. 27-29) and a downstream plate assembly 250(details shown in FIGS. 25-32), with a concentrating chamber 262 locatedbetween the upstream plate assembly 248 and the downstream plateassembly 250.

[0130] Through the upstream plate assembly 248 are a plurality ofvertically extending inlet slits 254. The downstream plate assembly 250includes a plurality of vertically extending exit slits 256 that are inalignment with the inlet slits 254. The exit slits 256 are, however,slightly wider than the inlet slits 254. The downstream plate assembly250 also includes flow channels 258 that extend substantially across thewidth of the entire downstream plate assembly 250, with each flowchannel 258 being adjacent to an excess gas withdrawal port 260.

[0131] During operation, the aerosol 108 passes through the inlet slits254 and into the concentrating chamber 262. Excess carrier gas 238 iswithdrawn from the concentrating chamber 262 via the excess gaswithdrawal ports 260. The withdrawn excess carrier gas 238 then exitsvia a gas duct port 264. That portion of the aerosol 108 that is notwithdrawn through the excess gas withdrawal ports 260 passes through theexit slits 256 and the flow channels 258 to form the concentratedaerosol 240. Those droplets passing across the concentrating chamber 262and through the exit slits 256 are those droplets of a large enough sizeto have sufficient momentum to resist being withdrawn with the excesscarrier gas 238.

[0132] As seen best in FIGS. 27-32, the inlet slits 254 of the upstreamplate assembly 248 include inlet nozzle extension portions 266 thatextend outward from the plate surface 268 of the upstream plate assembly248. The exit slits 256 of the downstream plate assembly 250 includeexit nozzle extension portions 270 extending outward from a platesurface 272 of the downstream plate assembly 250. These nozzle extensionportions 266 and 270 are important for operation of the virtual impactor246, because having these nozzle extension portions 266 and 270 permitsa very close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238.

[0133] Also as best seen in FIGS. 27-32, the inlet slits 254 have widthsthat flare outward toward the side of the upstream plate assembly 248that is first encountered by the aerosol 108 during operation. Thisflared configuration reduces the sharpness of surfaces encountered bythe aerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

[0134] As noted previously, both the inlet slits 254 of the upstreamplate assembly 248 and the exit slits 256 of the downstream plateassembly 250 are vertically extending. This configuration isadvantageous for permitting liquid that may collect around the inletslits 254 and the exit slits 256 to drain away. The inlet slits 254 andthe exit slits 256 need not, however, have a perfectly verticalorientation. Rather, it is often desirable to slant the slits backward(sloping upward and away in the direction of flow) by about five to tendegrees relative to vertical, to enhance draining of liquid off of theupstream plate assembly 248 and the downstream plate assembly 250. Thisdrainage function of the vertically extending configuration of the inletslits 254 and the outlet slits 256 also inhibits liquid build-up in thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

[0135] As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

[0136] Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

[0137] Any suitable droplet classifier may be used for removing dropletsabove a predetermined size. For example, a cyclone could be used toremove over-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34-38.

[0138] As seen in FIG. 34, an impactor 288 has disposed in a flowconduit 286 a flow control plate 290 and an impactor plate assembly 292.The flow control plate 290 is conveniently mounted on a mounting plate294.

[0139] The flow control plate 290 is used to channel the flow of theaerosol stream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

[0140] Details of the mounting plate 294 are shown in FIG. 36. Themounting plate 294 has a mounting flange 298 with a large diameter flowopening 300 passing therethrough to permit access of the aerosol 108 tothe flow ports 296 of the flow control plate 290 (shown in FIG. 35).

[0141] Referring now to FIGS. 37 and 38, one embodiment of an impactorplate assembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

[0142] During operation of the impactor 288, the aerosol 108 from theaerosol generator 106 passes through the upstream flow control plate290. Most of the droplets in the aerosol navigate around the impactorplate 302 and exit the impactor 288 through the downstream flow controlplate 290 in the classified aerosol 282. Droplets in the aerosol 108that are too large to navigate around the impactor plate 302 will impacton the impactor plate 302 and drain through the drain 296 to becollected with the drained liquid 284 (as shown in FIG. 34).

[0143] The configuration of the impactor plate 302 shown in FIG. 33represents only one of many possible configurations for the impactorplate 302. For example, the impactor 288 could include an upstream flowcontrol plate 290 having vertically extending flow slits therethroughthat are offset from vertically extending flow slits through theimpactor plate 302, such that droplets too large to navigate the changein flow due to the offset of the flow slits between the flow controlplate 290 and the impactor plate 302 would impact on the impactor plate302 to be drained away. Other designs are also possible.

[0144] In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm in size, more preferably to removedroplets larger than about 10 μm in size, even more preferably to removedroplets of a size larger than about 8 μm in size and most preferably toremove droplets larger than about 5 μm in size. The dropletclassification size in the droplet classifier is preferably smaller thanabout 15 μm, more preferably smaller than about 10 μm, even morepreferably smaller than about 8 μm and most preferably smaller thanabout 5 μm. The classification size, also called the classification cutpoint, is that size at which half of the droplets of that size areremoved and half of the droplets of that size are retained. Dependingupon the specific application, however, the droplet classification sizemay be varied, such as by changing the spacing between the impactorplate 302 and the flow control plate 290 or increasing or decreasingaerosol velocity through the jets in the flow control plate 290. Becausethe aerosol generator 106 of the present invention initially produces ahigh quality aerosol 108, having a relatively narrow size distributionof droplets, typically less than about 30 weight percent of liquid feed102 in the aerosol 108 is removed as the drain liquid 284 in the dropletclassifier 288, with preferably less than about 25 weight percent beingremoved, even more preferably less than about 20 weight percent beingremoved and most preferably less than about 15 weight percent beingremoved. Minimizing the removal of liquid feed 102 from the aerosol 108is particularly important for commercial applications to increase theyield of high quality particulate product 116. It should be noted,however, that because of the superior performance of the aerosolgenerator 106, it is frequently not required to use an impactor or otherdroplet classifier to obtain a desired absence of oversize droplets tothe furnace. This is a major advantage, because the added complexity andliquid losses accompanying use of an impactor may often be avoided withthe process of the present invention.

[0145] Sometimes it is desirable to use both the aerosol concentrator236 and the droplet classifier 280 to produce an extremely high qualityaerosol stream for introduction into the furnace for the production ofparticles of highly controlled size and size distribution. Referring nowto FIG. 39, one embodiment of the present invention is shownincorporating both the virtual impactor 246 and the impactor 288. Basiccomponents of the virtual impactor 246 and the impactor 288, as shown inFIG. 39, are substantially as previously described with reference toFIGS. 26-38. As seen in FIG. 39, the aerosol 108 from the aerosolgenerator 106 is fed to the virtual impactor 246 where the aerosolstream is concentrated to produce the concentrated aerosol 240. Theconcentrated aerosol 240 is then fed to the impactor 288 to remove largedroplets therefrom and produce the classified aerosol 282, which maythen be fed to the furnace 110. Also, it should be noted that by usingboth a virtual impactor and an impactor, both undesirably large andundesirably small droplets are removed, thereby producing a classifiedaerosol with a very narrow droplet size distribution. Also, the order ofthe aerosol concentrator and the aerosol classifier could be reversed,so that the aerosol concentrator 236 follows the aerosol classifier 280.

[0146] One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

[0147] With some applications of the process of the present invention,it may be possible to collect the particles 112 directly from the outputof the furnace 110. More often, however, it will be desirable to coolthe particles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114;traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

[0148] Referring now to FIGS. 41-43, one embodiment of a gas quenchcooler 330 is shown. The gas quench cooler includes a perforated conduit332 housed inside of a cooler housing 334 with an annular space 336located between the cooler housing 334 and the perforated conduit 332.In fluid communication with the annular space 336 is a quench gas inletbox 338, inside of which is disposed a portion of an aerosol outletconduit 340. The perforated conduit 332 extends between the aerosoloutlet conduit 340 and an aerosol inlet conduit 342. Attached to anopening into the quench gas inlet box 338 are two quench gas feed tubes344. Referring specifically to FIG. 43, the perforated tube 332 isshown. The perforated tube 332 has a plurality of openings 345. Theopenings 345, when the perforated conduit 332 is assembled into the gasquench cooler 330, permit the flow of quench gas 346 from the annularspace 336 into the interior space 348 of the perforated conduit 332.Although the openings 345 are shown as being round holes, any shape ofopening could be used, such as slits. Also, the perforated conduit 332could be a porous screen. Two heat radiation shields 347 preventdownstream radiant heating from the furnace. In most instances, however,it will not be necessary to include the heat radiation shields 347,because downstream radiant heating from the furnace is normally not asignificant problem. Use of the heat radiation shields 347 is notpreferred due to particulate losses that accompany their use.

[0149] With continued reference to FIGS. 41-43, operation of the gasquench cooler 330 will now be described. During operation, the particles112, carried by and dispersed in a gas stream, enter the gas quenchcooler 330 through the aerosol inlet conduit 342 and flow into theinterior space 348 of perforated conduit 332. Quench gas 346 isintroduced through the quench gas feed tubes 344 into the quench gasinlet box 338. Quench gas 346 entering the quench gas inlet box 338encounters the outer surface of the aerosol outlet conduit 340, forcingthe quench gas 346 to flow, in a spiraling, swirling manner, into theannular space 336, where the quench gas 346 flows through the openings345 through the walls of the perforated conduit 332. Preferably, the gas346 retains some swirling motion even after passing into the interiorspace 348. In this way, the particles 112 are quickly cooled with lowlosses of particles to the walls of the gas quench cooler 330. In thismanner, the quench gas 346 enters in a radial direction into theinterior space 348 of the perforated conduit 332 around the entireperiphery, or circumference, of the perforated conduit 332 and over theentire length of the perforated conduit 332. The cool quench gas 346mixes with and cools the hot particles 112, which then exit through theaerosol outlet conduit 340 as the cooled particle stream 322. The cooledparticle stream 322 can then be sent to the particle collector 114 forparticle collection. The temperature of the cooled particle stream 322is controlled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

[0150] Because of the entry of quench gas 346 into the interior space348 of the perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

[0151] As seen in FIGS. 41-43, the gas quench cooler 330 includes a flowpath for the particles 112 through the gas quench cooler of asubstantially constant cross-sectional shape and area. Preferably, theflow path through the gas quench cooler 330 will have the samecross-sectional shape and area as the flow path through the furnace 110and through the conduit delivering the aerosol 108 from the aerosolgenerator 106 to the furnace 110. In one embodiment, however, it may benecessary to reduce the cross-sectional area available for flow prior tothe particle collector 114. This is the case, for example, when theparticle collector includes a cyclone for separating particles in thecooled particle stream 322 from gas in the cooled particle stream 322.This is because of the high inlet velocity requirements into cycloneseparators.

[0152] Referring now to FIG. 44, one embodiment of the gas quench cooler330 is shown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

[0153] In an additional embodiment, the process of the present inventioncan also incorporate compositional modification of the particles 112exiting the furnace. Most commonly, the compositional modification willinvolve forming on the particles 112 a material phase that is differentthan that of the particles 112, such as by coating the particles 112with a coating material. One embodiment of the process of the presentinvention incorporating particle coating is shown in FIG. 45. As shownin FIG. 45, the particles 112 exiting from the furnace 110 go to aparticle coater 350 where a coating is placed over the outer surface ofthe particles 112 to form coated particles 352, which are then sent tothe particle collector 114 for preparation of the particulate product116. Coating methodologies employed in the particle coater 350 arediscussed in more detail below.

[0154] With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route. When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

[0155] In a further embodiment of the present invention, followingpreparation of the particles 112 in the furnace 110, the particles 112may then be structurally modified to impart desired physical propertiesprior to particle collection. Referring now to FIG. 46, one embodimentof the process of the present invention is shown including suchstructural particle modification. The particles 112 exiting the furnace110 go to a particle modifier 360 where the particles are structurallymodified to form modified particles 362, which are then sent to theparticle collector 114 for preparation of the particulate product 116.The particle modifier 360 is typically a furnace, such as an annealingfurnaces which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

[0156] The structural modification that occurs in the particle modifier360 may be any modification to the crystalline structure or morphologyof the particles 112. For example, the particles 112 may be annealed inthe particle modifier 360 to densify the particles 112 or torecrystallize the particles 112 into a polycrystalline or singlecrystalline form. Also, especially in the case of composite particles112, the particles may be annealed for a sufficient time to permitredistribution within the particles 112 of different material phases.Particularly preferred parameters for such processes are discussed inmore detail below.

[0157] The initial morphology of composite particles made in the furnace110, according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

[0158] Referring now to FIG. 48, an embodiment of the apparatus of thepresent invention is shown that includes the aerosol generator 106 (inthe form of the 400 transducer array design), the aerosol concentrator236 (in the form of a virtual impactor), the droplet classifier 280 (inthe form of an impactor), the furnace 110, the particle cooler 320 (inthe form of a gas quench cooler) and the particle collector 114 (in theform of a bag filter). All process equipment components are connectedvia appropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

[0159] Aerosol generation with the process of the present invention hasthus far been described with respect to the ultrasonic aerosolgenerator. Use of the ultrasonic generator is preferred for the processof the present invention because of the extremely high quality and denseaerosol generated. In some instances, however, the aerosol generationfor the process of the present invention may have a different designdepending upon the specific application. For example, when largerparticles are desired, such as those having a weight average size oflarger than about 3 μm, a spray nozzle atomizer may be preferred. Forsmaller-particle applications, however, and particularly for thoseapplications to produce particles smaller than about 3 μm, andpreferably smaller than about 2 μm in size, as is generally desired withthe particles of the present invention, the ultrasonic generator, asdescribed herein, is particularly preferred. In that regard, theultrasonic generator of the present invention is particularly preferredfor when making particles with a weight average size of from about 0.2μm to about 3 μm.

[0160] Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5-24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

[0161] Through the careful and controlled design of the ultrasonicgenerator of the present invention, an aerosol may be produced typicallyhaving greater than about 70 weight percent (and preferably greater thanabout 80 weight percent) of droplets in the size range of from about 1μm to about 10 μm, preferably in a size range of from about 1 μm toabout 5 μm and more preferably from about 2 μm to about 4 μm. Also, theultrasonic generator of the present invention is capable of deliveringhigh output rates of liquid feed in the aerosol. The rate of liquidfeed, at the high liquid loadings previously described, is preferablygreater than about 25 milliliters per hour per transducer, morepreferably greater than about 37.5 milliliters per hour per transducer,even more preferably greater than about 50 milliliters per hour pertransducer and most preferably greater than about 100 millimeters perhour per transducer. This high level of performance is desirable forcommercial operations and is accomplished with the present inventionwith a relatively simple design including a single precursor bath overan array of ultrasonic transducers. The ultrasonic generator is made forhigh aerosol production rates at a high droplet loading, and with anarrow size distribution of droplets. The generator preferably producesan aerosol at a rate of greater than about 0.5 liter per hour ofdroplets, more preferably greater than about 2 liters per hour ofdroplets; still more preferably greater than about 5 liters per hour ofdroplets, even more preferably greater than about 10 liters per hour ofdroplets and most preferably greater than about 40 liters per hour ofdroplets. For example, when the aerosol generator has a 400 transducerdesign, as described with reference to FIGS. 7-24, the aerosol generatoris capable of producing a high quality aerosol having high dropletloading as previously described, at a total production rate ofpreferably greater than about 10 liters per hour of liquid feed, morepreferably greater than about 15 liters per hour of liquid feed, evenmore preferably greater than about 20 liters per hour of liquid feed andmost preferably greater than about 40 liters per hour of liquid feed.

[0162] Under most operating conditions, when using such an aerosolgenerator, total particulate product produced is preferably greater thanabout 0.5 gram per hour per transducer, more preferably greater thanabout 0.75 gram per hour per transducer, even more preferably greaterthan about 1.0 gram per hour per transducer and most preferably greaterthan about 2.0 grams per hour per transducer.

[0163] One significant aspect of the process of the present inventionfor manufacturing particulate materials is the unique flowcharacteristics encountered in the furnace relative to laboratory scalesystems. The maximum Reynolds number attained for flow in the furnace110 with the present invention is very high, typically in excess of 500,preferably in excess of 1,000 and more preferably in excess of 2,000. Inmost instances, however, the maximum Reynolds number for flow in thefurnace will not exceed 10,000, and preferably will not exceed 5,000.This is significantly different from lab-scale systems where theReynolds number for flow in a reactor is typically lower than 50 andrarely exceeds 100.

[0164] The Reynolds number is a dimensionless quantity characterizingflow of a fluid which, for flow through a circular cross sectionalconduit is defined as: ${Re} = \frac{\rho \quad {vd}}{\mu}$

[0165] where:

[0166] ρ=fluid density;

[0167] v=fluid mean velocity;

[0168] d=conduit inside diameter; and

[0169] μ=fluid viscosity.

[0170] It should be noted that the values for density, velocity andviscosity will vary along the length of the furnace 110. The maximumReynolds number in the furnace 110 is typically attained when theaverage stream temperature is at a maximum, because the gas velocity isat a very high value due to gas expansion when heated.

[0171] One problem with operating under flow conditions at a highReynolds number is that undesirable volatilization of components is muchmore likely to occur than in systems having flow characteristics asfound in laboratory-scale systems. The volatilization problem occurswith the present invention, because the furnace is typically operatedover a substantial section of the heating zone in a constant wall heatflux mode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

[0172] With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired; the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

[0173] Therefore, with the present invention, it is preferred that whenthe flow characteristics in the furnace are such that the Reynoldsnumber through any part of the furnace exceeds 500, more preferablyexceeds 1,000, and most preferably exceeds 2,000, the maximum walltemperature in the furnace should be kept at a temperature that is belowthe temperature at which a desired component of the final particleswould exert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

[0174] In addition to maintaining the furnace wall temperature below alevel that could create volatilization problems, it is also importantthat this not be accomplished at the expense of the desired averagestream temperature. The maximum average stream temperature must bemaintained at a high enough level so that the particles will have adesired high density. The maximum average stream temperature should,however, generally be a temperature at which a component in the finalparticles, or an intermediate component from which a component in thefinal particles is at least partially derived, would exert a vaporpressure not exceeding about 100 millitorr, preferably not exceedingabout 50 millitorr, and most preferably not exceeding about 25millitorr.

[0175] So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second, and mostpreferably shorter than about 0.2 second.

[0176] Another significant issue with respect to operating the processof the present invention, which includes high aerosol flow rates, isloss within the system of materials intended for incorporation into thefinal particulate product. Material losses in the system can be quitehigh if the system is not properly operated. If system losses are toohigh, the process would not be practical for use in the manufacture ofparticulate products of many materials. This has typically not been amajor consideration with laboratory-scale systems.

[0177] One significant potential for loss with the process of thepresent invention is thermophoretic losses that occur when a hot aerosolstream is in the presence of a cooler surface. In that regard, the useof the quench cooler, as previously described, with the process of thepresent invention provides an efficient way to cool the particleswithout unreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

[0178] It has been found that thermophoretic losses in the back end ofthe furnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

[0179] Typically, it would be desirable to instantaneously cool theaerosol upon exiting the furnace. This is not possible. It is possible,however, to make the residence time between the furnace outlet and thecooling unit as short as possible. Furthermore, it is desirable toinsulate the aerosol conduit occurring between the furnace exit and thecooling unit entrance. Even more preferred is to insulate that conduitand, even more preferably, to also heat that conduit so that the walltemperature of that conduit is at least as high as the average streamtemperature of the aerosol stream. Furthermore, it is desirable that thecooling unit operate in a manner such that the aerosol is quickly cooledin a manner to prevent thermophoretic losses during cooling. The quenchcooler, described previously, is very effective for cooling with lowlosses. Furthermore, to keep the potential for thermophoretic lossesvery low, it is preferred that the residence time of the aerosol streambetween attaining the maximum stream temperature in the furnace and apoint at which the aerosol has been cooled to an average streamtemperature below about 200° C. is shorter than about 2 seconds, morepreferably shorter than about 1 second, and even more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.In most instances, the maximum average stream temperature attained inthe furnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace toa point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

[0180] Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

[0181] To reduce the potential for thermophoretic losses before theparticles are finally collected, it is important that the transitionbetween the cooling unit and particle collection be as short aspossible. Preferably, the output from the quench cooler is immediatelysent to a particle separator, such as a filter unit or a cyclone. Inthat regard, the total residence time of the aerosol between attainingthe maximum average stream temperature in the furnace and the finalcollection of the particles is preferably shorter than about 2 seconds,more preferably shorter than about 1 second, still more preferablyshorter than about 0.5 second and most preferably shorter than about 0.1second. Furthermore, the residence time between the beginning of theheating zone in the furnace and final collection of the particles ispreferably shorter than about 6 seconds, more preferably shorter thanabout 3 seconds, even more preferably shorter than about 2 seconds, andmost preferably shorter than about 1 second. Furthermore, the potentialfor thermophoretic losses may further be reduced by insulating theconduit section between the cooling unit and the particle collector and,even more preferably, by also insulating around the filter, when afilter is used for particle collection. The potential for losses may bereduced even further by heating of the conduit section between thecooling unit and the particle collection equipment, so that the internalequipment surfaces are at least slightly warmer than the aerosol streamaverage stream temperature. Furthermore, when a filter is used forparticle collection, the filter could be heated. For example, insulationcould be wrapped around a filter unit, with electric heating inside ofthe insulating layer to maintain the walls of the filter unit at adesired elevated temperature higher than the temperature of filterelements in the filter unit, thereby reducing thermophoretic particlelosses to walls of the filter unit.

[0182] Even with careful operation to reduce thermophoretic losses, somelosses will still occur. For example, some particles will inevitably belost to walls of particle collection equipment, such as the walls of acyclone or filter housing. One way to reduce these losses, andcorrespondingly increase product yield, is to periodically wash theinterior of the particle collection equipment to remove particlesadhering to the sides. In most cases, the wash fluid will be water,unless water would have a detrimental effect on one of the components ofthe particles. For example, the particle collection equipment couldinclude parallel collection paths. One path could be used for activeparticle collection while the other is being washed. The wash couldinclude an automatic or manual flush without disconnecting theequipment. Alternatively, the equipment to be washed could bedisconnected to permit access to the interior of the equipment for athorough wash. As an alternative to having parallel collection paths,the process could simply be shut down occasionally to permitdisconnection of the equipment for washing. The removed equipment couldbe replaced with a clean piece of equipment and the process could thenbe resumed while the disconnected equipment is being washed.

[0183] For example, a cyclone or filter unit could periodically bedisconnected and particles adhering to interior walls could be removedby a water wash. The particles could then be dried in a low temperaturedryer, typically at a temperature of lower than about 50° C.

[0184] In one embodiment, wash fluid used to wash particles from theinterior walls of particle collection equipment includes a surfactant.Some of the surfactant will adhere to the surface of the particles. Thiscould be advantageous to reduce agglomeration tendency of the particlesand to enhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

[0185] Another area for potential losses in the system, and for theoccurrence of potential operating problems, is between the outlet of theaerosol generator and the inlet of the furnace. Losses here are not dueto thermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains, as previously described. In that regard, it ispreferred that a drain be placed as close as possible to the furnaceinlet to prevent liquid accumulations from reaching the furnace. Thedrain should be placed, however, far enough in advance of the furnaceinlet such that the stream temperature is lower than about 80° C. at thedrain location.

[0186] Another way to reduce the potential for undesirable liquidbuildup is for the conduit between the aerosol generator outlet and thefurnace inlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

[0187] Another way to reduce the potential for undesirable buildup is toheat at least a portion, and preferably the entire length, of theconduit between the aerosol generator and the inlet to the furnace. Forexample, the conduit could be wrapped with a heating tape to maintainthe inside walls of the conduit at a temperature higher than thetemperature of the aerosol. The aerosol would then tend to concentratetoward the center of the conduit due to thermophoresis. Fewer aerosoldroplets would, therefore, be likely to impinge on conduit walls orother surfaces making the transition to the furnace.

[0188] Another way to reduce the potential for undesirable liquidbuildup is to introduce a dry gas into the aerosol between the aerosolgenerator and the furnace. Referring now to FIG. 49, one embodiment ofthe process is shown for adding a dry gas 118 to the aerosol 108 beforethe furnace 110. Addition of the dry gas 118 causes vaporization of atleast a part of the moisture in the aerosol 108, and preferablysubstantially all of the moisture in the aerosol 108, to form a driedaerosol 119, which is then introduced into the furnace 110.

[0189] The dry gas 118 will most often be dry air, although in someinstances it may be desirable to use dry nitrogen gas or some other drygas if sufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 41-43, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

[0190] Still another way to reduce the potential for losses due toliquid accumulation is to operate the process with equipmentconfigurations such that the aerosol stream flows in a verticaldirection from the aerosol generator to and through the furnace. Forsmaller-size particles, those smaller than about 1.5 μm, this verticalflow should, preferably, be vertically upward. For larger-sizeparticles, such as those larger than about 1.5 μm, the vertical flow ispreferably vertically downward.

[0191] Furthermore; with the process of the present invention, thepotential for system losses is significantly reduced because the totalsystem retention time from the outlet of the generator until collectionof the particles is typically shorter than about 10 seconds, preferablyshorter than about 7 seconds, more preferably shorter than about 5seconds and most preferably shorter than about 3 seconds.

[0192] In accordance with the foregoing methodology for the productionof sulfur-containing phosphors, the liquid feed includes the chemicalcomponents that will form the sulfur-containing phosphor particles,including the activator ions. The sulfur-containing phosphor precursormay be a substance in either a liquid or solid phase of the liquid feed.Typically, the sulfur-containing phosphor precursor will be a metal saltdissolved in a sulfur-containing acid. The sulfur-containing phosphorprecursor may undergo one or more chemical reactions in the furnace toassist in production of the particles. Alternatively, thesulfur-containing phosphor precursor may contribute to the formation ofthe particles without undergoing chemical reaction. This could be thecase, for example, when the liquid feed includes suspended particles asa precursor material. According to one embodiment of the presentinvention, the precursor undergoes conversion to an intermediateproduct, which is then treated to convert it into the sulfur-containingphosphor.

[0193] Metal sulfide phosphors (MS:M′) can be prepared from an aqueoussolution by the reaction of a metal compound such as a carbonate, oxide,hydroxide, sulfate or nitrate with a sulfur-containing acid such asthioacetic acid, thiocarboxylic acid (HS(O)CR) or dithiocarboxylic acid,to form a water soluble complex, such as M(S(O)CR)₂xH₂O. The complex canalso be formed from a soluble metal salt and sulfur-containing ligandsuch as thiourea. Similar precursors can be used for the activator ion.Preferably, at least about 2 equivalents of acid are added to ensurecomplete reaction with the metal compound. The solution, when pyrolyzedunder N₂, leads to the metal sulfide.

MCO₃+2HS(O)CR—H₂O->M(S(O)CR)₂ ⁻xH₂O+CO₂+H₂O

M(S(O)CR)₂ ^(−x)H₂O+heat/N₂ MS+volatile by-products

MSO₄→MS+volatile by-products

M(NO₃)₂+SC(NR₂)₂ MS+volatile by-products

M(SCNR₂)₂→MS+volatile by-products

[0194] The solution preferably has a phosphor precursor concentrationthat is unsaturated to avoid the formation of precipitates andpreferably includes sufficient precursor to yield from about 1 to about50 weight percent, such as from about 1 to 15 weight percent, of thephosphor compound, based on the total amount of metal(s) in solution.Preferably the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable for specificmaterials. The use of organic solvents can, however, lead to undesirablecarbon contamination in the phosphor particles. The pH of theaqueous-based solutions can be adjusted to alter the solubilitycharacteristics of the precursor in the solution.

[0195] In addition to the host material, the liquid feed preferablyincludes the precursor to the activator ion. For example, for theproduction of ZnS:Mn phosphor particles, the precursor solutionpreferably includes a zinc precursor such as zinc nitrate as well asmanganese carbonate. The relative concentrations of the precursors canbe easily adjusted to vary the concentration of the activator ion in thehost material.

[0196] In addition to the foregoing, the liquid feed can also includeother additives that contribute to the formation of the particles. Forexample, a fluxing agent can be added to the solution to increase thecrystallinity and/or density of the particles. For example, the additionof urea to metal salt solutions, such as a metal nitrate, can increasethe density of particles produced from the solution. In one embodiment,up to about 1 mole equivalent urea is added to the precursor solution,as measured against the moles of phosphor compound in the metal saltsolution. Further, if the particles are to be coated phosphor particles,as is discussed in more detail below., a soluble precursor to both thesulfur-containing phosphor compound and the coating can be used in theprecursor solution wherein the coating precursor is an involatile orvolatile species.

[0197] For the production of sulfur-containing phosphor particles, thecarrier gas may comprise any gaseous medium in which droplets producedfrom the liquid feed may be dispersed in aerosol form. Also, the carriergas may be inert, in that the carrier gas does not participate information of the phosphor particles. Alternatively, the carrier gas mayhave one or more active component(s) that contribute to formation of thephosphor particles. In that regard, the carrier gas may include one ormore reactive components that react in the furnace to contribute toformation of the phosphor particles. In many applications, air will be asatisfactory carrier gas. In other instances, a relatively inert gassuch as nitrogen may be required. An inert gas would sometimes beuseful, for example, when preparing particles comprising sulfidematerials or other materials that are free of oxygen.

[0198] When the particles are modified by coating the particles, as isdiscussed above, the precursors to metal oxide coatings can be selectedfrom volatile metal acetates, chlorides, alkoxides or halides. Suchprecursors are known to react at high temperatures to form thecorresponding metal oxides and eliminate supporting ligands or ions. Forexample, SiCl₄ can be used as a precursor to SiO₂ coatings when watervapor is present:

SiCl₄₍ g)+2H₂O_((g))------->SiO_(SiO) _(2(s))+4HCl_((g))

[0199] SiCl₄ also is highly volatile and is a liquid at roomtemperature, which makes transport into the reactor more controllable.

[0200] Metal alkoxides can be used to produce metal oxide films byhydrolysis. The water molecules react with the alkoxide M-O bondresulting in clean elimination of the corresponding alcohol with theformation of M-O-M bonds:

Si(OEt)₄+2H₂O--------->SiO₂+4EtOH

[0201] Most metal alkoxides have a reasonably high vapor pressure andare therefore well suited as coating precursors.

[0202] Metal acetates are also useful as coating precursors since theyreadily decompose upon thermal activation by acetic anhydrideelimination:

Mg(O₂CCH₃)₂---------->MgO+CH₃C(O)OC(O)CH₃

[0203] Metal acetates are advantageous as coating precursors since theyare water stable and are reasonably inexpensive.

[0204] Coatings can be generated on the particle surface by a number ofdifferent mechanisms. One or more precursors can vaporize and fuse tothe hot phosphor particle surface and thermally react resulting in theformation of a thin-film coating by chemical vapor deposition (CVD).Preferred coatings deposited by CVD include metal oxides and elementalmetals. Further, the coating can be formed by physical vapor deposition(PVD) wherein a coating material physically deposits an the surface ofthe particles. Preferred coatings deposited by PVD include organicmaterials and elemental metal. Alternatively, the gaseous precursor canreact in the gas phase forming small particles, for example less thanabout 5 nanometers in size, which then diffuse to the larger particlesurface and sinter onto the surface, thus forming a coating. This methodis referred to as gas-to-particle conversion (GPC). Whether such coatingreactions occur by CVD, PVD or GPC is dependent on the reactorconditions such as precursor partial pressure, water partial pressureand the concentration of particles in the gas stream. Another possiblesurface coating method is surface conversion of the surface of theparticle by reaction with a vapor phase reactant to convert the surfaceof the particles to a different material than that originally containedin the particles.

[0205] In addition, a volatile coating material such as PbO, MoO₃ orV₂O₅ can be introduced into the reactor such that the coating depositson the particle by condensation. Highly volatile metals, such as silver,can also be deposited by condensation. Further, the phosphor powders canbe coated using other techniques. For example, a soluble precursor toboth the phosphor powder and the coating can be used in the precursorsolution wherein the coating precursor is involatile (e.g. Al(NO₃)₃) orvolatile (e.g. Sn(OAc)₄ where AC is acetate). In another method, acolloidal precursor and a soluble phosphor precursor can be used to forma particulate colloidal coating on the phosphor.

[0206] The structural modification that occurs in the particle modifiermay be any modification to the crystalline structure or morphology ofthe particles. For example, the particles can be annealed in theparticle modifier to densify the particles or to recrystallize theparticles into a polycrystalline or single crystalline form. Also,especially in the case of composite particles, the particles may beannealed for a sufficient time to permit redistribution within theparticles of different material phases or permit redistribution of theactivator ion(s).

[0207] More specifically, while the sulfur-containing phosphor powdersproduced by the foregoing method have good crystallinity, it may bedesirable to increase the crystallinity (average crystallite size) afterproduction. Thus, the powders can be annealed (heated) for an amount oftime and in a preselected environments to increase the crystallinity ofthe phosphor particles. Increased crystallinity can advantageously yieldan increased brightness and efficiency of the phosphor particles. Ifsuch annealing steps are performed, the annealing temperature and timeshould be selected to minimize the amount of interparticle sinteringthat is often associated with annealing. According to one embodiment ofthe present invention, the sulfur-containing phosphor powder ispreferably annealed at a temperature of from about 700° C. to about1100° C., more preferably from about 800° C. to about 1000° C. Theannealing time is preferably not more than about 2 hours and can be aslow as about 1 minute. The sulfur-containing powders are typicallyannealed in an inert gas, such as argon.

[0208] Further, the crystallinity of the phosphors can be increased byusing a fluxing agent, either in the precursor solution or in apost-formation annealing step. A fluxing agent is a reagent whichimproves the crystallinity of the material when the reagent and thematerial are heated together, as compared to heating the material to thesame temperature and for the same amount of time in the absence of thefluxing agent. The fluxing agents typically cause a eutectic to formwhich leads to a liquid phase at the grain boundaries, increasing thediffusion coefficient. The fluxing agent, for example alkali metalhalides such as NaCl or KCl or an organic compound such as urea(CO(NH₂)₂), can be added to the precursor solution where it improves thecrystallinity of the particles during their subsequent formation.Alternatively, the fluxing agent can be contacted with the phosphorpowder batches after they have been collected. Upon annealing, thefluxing agent improves the crystallinity and/or density of the phosphorpowder, and therefore improves other properties such as the brightnessof the phosphor powder. Also, in the case of composite particles 112,the particles may be annealed for a sufficient time to permitredistribution within the particles 112 of different material phases.

[0209] Thus, the present invention is particularly applicable tosulfur-containing phosphor compounds. Phosphors are materials which arecapable of emitting radiation in the visible or ultraviolet spectralrange upon excitation, such as excitation by an external electric fieldor other external energy source. Sulfur-containing phosphors are aparticular class of phosphor compounds that have a host material thatincludes sulfur. More particularly, such sulfur-containing phosphorsaccording to the present invention include metal sulfides, oxysulfidesand thiogallates. The sulfur-containing phosphor particles of thepresent invention can be chemically tailored to emit specificwavelengths of visible light, such as red, blue or green light and bydispersing different phosphor powders in a predetermined arrangement andcontrollably exciting the powders, a full-color visual display can beproduced.

[0210] Sulfur-containing phosphors include a matrix compound, referredto as a host material, and the phosphor further includes one or moredopants, referred to as activator ions, to emit a specific color or toenhance the luminescence characteristics. Some phosphors, such asup-convertor phosphors, incorporate more than one activator ion.

[0211] Phosphors can be classified by their phosphorescent propertiesand the present invention is applicable to all types of these phosphors.For example, electroluminescent phosphors are phosphors that emit lightupon stimulation by an electric field. These phosphors are used forthin-film and thick-film electroluminescent displays, back lighting forLCD's and electroluminescent lamps used in wrist watches and the like.Cathodoluminescent phosphors emit light upon stimulation by electronbombardment. These phosphors are utilized in CRT's (e.g. commontelevisions) and FED's.

[0212] Photoluminescent phosphors emit light upon-stimulation by otherlight. The stimulating light usually has higher energy than the emittedlight. For example, a photoluminescent phosphor can emit visible lightwhen stimulated by ultraviolet light. These phosphors are utilized inplasma display panels and common fluorescent lamps.

[0213] Up-converter phosphors also emit light upon stimulation by otherlight, but usually light of a lower energy than the emitted light. Forexample, infrared light can be used to stimulate an up-converterphosphor which then emits visible or ultraviolet light. Up-convertorphosphors typically include at least 2 activator ions which convert thelower energy infrared light. These phosphor materials are useful inimmunoassay and security applications. Similarly, x-ray phosphors areutilized to convert x-rays to visible light and are useful in medicaldiagnostics.

[0214] The sulfur-containing host material can be doped with anactivator ion in an amount which is sufficient for a particularapplication. Preferably, the activator ion is incorporated in an amountof from about 0.02 to about 15 atomic percent, more preferably fromabout 0.02 to about 10 atomic percent and even more preferably fromabout 0.02 to about 5 atomic percent. It will be appreciated, as isdiscussed in more detail below, that the preferred concentration of theactivator ion(s) in the host material can vary for differentapplications.

[0215] One advantage of the present invention is that the activator ionis homogeneously distributed throughout the host material. Phosphorpowders prepared by solid-state methods do not give uniformconcentration of the activator ion in small particles and solutionroutes also do not give homogenous distribution of the activator ion dueto different rates of precipitation.

[0216] Particular sulfur-containing phosphor compounds may be mostuseful for certain applications and no single compound is necessarilypreferred for all possible applications. However, preferredsulfur-containing phosphor host materials for some display applicationsinclude the metal sulfides, particularly the Group 2 metal sulfides(e.g. CaS, SrS, BaS and MgS) and the Group 12 metal sulfides (e.g. ZnSand CdS). For such metal sulfides, preferred activator ions can beselected from the rare-earth elements (e.g. La, Ce, Pm, Eu, Gd, Tb, andYb), preferably Eu or Tb, particularly for Group 2 metal sulfides. Theacivator ion can also be selected from Cu, Mn, Ag, Al, Au, Ga and Cl.Mixtures of these activator ions can advantageously be used,particularly for up-convertor phosphors.

[0217] ZnS is particularly preferred for many cathodoluminescent displayapplications, particularly those utilizing high voltages (i.e. greaterthan about 2000 volts), due primarily to the high brightness of ZnS. ZnSis typically doped with Cu, Ag, Al, Au, Cl or mixtures thereof. Forexample, ZnS:Ag is a common cathodoluminescent phosphor used to produceblue light in a CRT device.

[0218] Many of the foregoing metal sulfide phosphors cannot easily beproduced using conventional techniques. Examples include CaS:Eu (red),SrS:Eu (orange), BaS:Ce (yellow), BaS:Tb (yellow), BaS:Mn(yellow-green), CaS:Tb (green-yellow), CaS:Ce (green) and SrS:Ce(blue-green). The methodology of the present invention advantageouslypermits such phosphor compounds to be produced with sufficientluminescent properties to be utilized in commercial devices.

[0219] In addition, the present invention provides the unique ability toproduce mixed-metal sulfides of the general form M¹ _(x)M² _(1-x)S,wherein M¹ and M² are Group 2 metals (e.g. Mg_(x)Sr_(1-x)S orCa_(x)Sr_(1-x)S) or wherein M¹ and M² are Group 12 metals (e.g.Zn_(x)Cd_(1-x)S). Complex mixed metal sulfides, for exampleBa_(x)Sr_(y)Ca_(1-x-y)S can also be produced. This unique feature of thepresent invention enables the formation of phosphors having luminescencecharacteristics that are selectively controllable. For example, anycolor from orange to red can be selected by varying the value of x inthe mixed metal sulfide Ca_(x)Sr_(1-x)S:Eu. Likewise, any color fromgreen to yellow can be selected by varying the value of x in the mixedmetal sulfide Ca_(x)Ba_(1-x)S:Ce and any color from blue-green to greencan be selected by varying the value of x in the mixed metal sulfideCa_(x)Sr_(1-x)S:Ce.

[0220] Other sulfur-containing phosphor compounds that can be producedaccording to the present invention include thiogallates of the formMGa₂S₄ wherein M can be Ca, Sr, Ba, Mg or mixtures thereof. Suchcompounds are typically doped with a rare-earth as an activator ion.Preferred examples include SrGa₂S₄:Eu (green), SrGa₂S₄:Ce (blue),CaGa₂S₄:Eu and CaGa₂S₄:Ce (blue-green). As with the mixed-metalsulfides, mixed metal thiogallates can be produced, such asCa_(x)Sr_(1-x)Ga₂S₄. Further, thiogallates include compounds whereinaluminum or indium substitute for gallium in the structure, such asCaAl_(x)Ga_(2-x)S₄CaIn_(x)Ga_(2-x)S₄, SrAl_(x)Ga_(2-x)S₄ orCaAl_(x)Ga_(2-x)S₄. The substitution of various amounts of aluminum orindium for gallium can advantageously adjust the chromaticity (color) ofthe phosphor compound.

[0221] In addition, oxysulfides, particularly Y₂O₂S:Eu and rare-earthoxysulfides such as Gd₂O₂S:Tb and La₂O₂S:Tb can also be produced inaccordance with the present invention. Some preferred sulfur-containingphosphor host materials and activator ions are listed in Table 1. TABLEI Examples of Sulfur-Containing Phosphors Host Material Activator IonColor BaS Ce Yellow CaS Ce Green CaS Mn Yellow SrS Ce Blue-GreenMg_(X)Sr_(1−X)S Ce Blue-Green ZnS Cu Blue-Green Y₂O₂S Eu Red SrGa₂S₄ EuGreen SrGa₂S₄ Ce Blue

[0222] The thiogallate phosphor compounds according to the presentinvention include, but re not limited to, thiogallates such as CaGa₂S₄Euor Ce and SrGa₂S₄Eu or Ce. Such thiogallate phosphors are useful in manyapplications, but are not believed to be widely available withsufficient luminescent properties for commercial applications due to thedifficulty producing such compounds. The reaction of strontium nitrateand gallium nitrate, when carried out in the solid state, does notresult in the formation of single phase SrGa₂S₄ because strontiumnitrate melts and segregates from the gallium nitrate. Thiogallates aredifficult to produce even using a standard spray pyrolysis techniquesince organic solvents are required, which can lead to powders havingincreased carbon contamination.

[0223] The thiogallate phosphor compounds according to the presentinvention are preferably produced using a process referred to herein asspray-conversion. Spray-conversion is a process wherein a spraypyrolysis technique, as is described in detail previously, is used toproduce an intermediate product, such as an oxide, that is capable ofbeing subsequently converted to the thiogallate. The intermediateproduct advantageously has many of the desirable morphological andchemical properties discussed hereinbelow, such as a small particle sizeand high purity.

[0224] For the production of thiogallates, water-soluble precursormaterials, such as nitrate salts, are placed into solution and areconverted at a low temperature, such as from about 700° C. to 800° C.,to a crystalline phase, such as the oxide phase MGa₂O₄ (where M can be,for example, Sr or Ca). The oxide phase is in the form of smallparticles having a narrow size distribution, as is described in moredetail below. The intermediate product is then converted by heating inthe presence of sulfur or a sulfur-containing compound, liquid or gas.For example, the powder can be admixed with sulfur or contacted with CS₂liquid. In a preferred embodiment, H₂S gas at an elevated temperature iscontacted with the intermediate product powder to form a substantiallyphase pure thiogallate having high crystallinity. The resulting powdercan be gently milled to remove any soft agglomerates that result fromthe heating process. The powder can also be annealed under an inert gasto increase the crystallinity of the powders, possibly in the presenceof a fluxing agent.

[0225] The resulting end product is a thiogallate powder having thedesirable morphological and luminescent properties. The average particlesize and morphological characteristics are primarily determined by thecharacteristics of the intermediate product.

[0226] Although discussed herein with reference to thiogallates, it willbe appreciated that other sulfur-containing phosphors, including ZnS,CdS, SrS or CaS, could be produced using a similar spray-conversionprocess. Thus, the precursors, such as nitrate salts, can bespray-converted at a temperature of, for example, 700° C. to 800° C. toform oxides or sulfides having low crystallinity. The intermediateproduct can then be roasted under H₂S gas at a temperature of, forexample, 800° C. to 1100° C., to form the metal sulfide phosphorcompounds. The phosphor particles can be further annealed to increasecrystallinity of the particles and can be lightly milled to removeagglomerates.

[0227] The powder characteristics that are preferred will depend uponthe application of the sulfur-containing phosphor powders. Nonetheless,it can be generally stated that the powders typically should have asmall average particle size, narrow size distribution, sphericalmorphology, high density and low porosity, high crystallinity and ahomogenous distribution of activator ion throughout the host material.The efficiency of the phosphor, defined as the overall conversion ofexcitation energy to visible photons, should be high.

[0228] According to the present invention, the sulfur-containingphosphor powder consists of particles having a small average particlesize. Although the preferred average size of the phosphor particles willvary according to the application of the phosphor powder, the averageparticle size of the phosphor particles is preferably not greater thanabout 10 μm. For most applications, the average particle size is morepreferably not greater than about 5 μm, such as from about 0.1 μm toabout 5 μm and more preferably is not greater than about 3 μm, such asfrom about 0.3 μm to about 3 μm. As used herein, the average particlesize is the weight average particle size.

[0229] According to the present invention, the powder batch of phosphorparticles also has a narrow particle size distribution, such that themajority of particles are substantially the same size. Preferably, atleast about 90 weight percent of the particles and more preferably atleast about 95 weight percent of the particles are not larger than twicethe average particle size. Thus, when the average particle size is about2 μm, it is preferred that at least about 90 weight percent of theparticles are not larger than 4 μm and it is more preferred that atleast about 95 weight percent of the particles are not larger than 4 μm.Further, it is preferred that at least about 90 weight percent of theparticles, and more preferably at least about 95 weight percent of theparticles, are not larger than about 1.5 times the average particlesize. Thus, when the average particle size is about 2 μm, it ispreferred that at least about 90 weight percent of the particles are notlarger than about 3 μm and it is more preferred that at least about 95weight percent of the particles are not larger than about 3 μm.

[0230] The phosphor particles of the present invention can besubstantially single crystal particles or may be comprised of a numberof crystallites. According to the present invention, the phosphorparticles are highly crystalline and it is preferred that the averagecrystallite size approaches the average particle size such that theparticles are mostly single crystals or are composed of only a few largecrystals. The average crystallite size of the particles is preferably atleast about 25 nanometers, more preferably is at least about 40nanometers, even more preferably is at least about 60 nanometers andmost preferably is at least about 80 nanometers. In one embodiment, theaverage crystallite size is at least about 100 nanometers. As it relatesto particle size, the average crystallite size is preferably at leastabout 20 percent, more preferably at least about 30 percent and mostpreferably is at least about 40 percent of the average particle size.Such highly crystalline phosphors are believed to have increasedluminescent efficiency and brightness as compared to phosphors havingsmaller crystallites.

[0231] The sulfur-containing phosphor particles of the present inventionadvantageously have a high degree of purity, that is, a low level ofimpurities. Impurities are materials that are not intended in the finalproduct. Thus, an activator ion is not considered to be an impurity. Thelevel of impurities in the phosphor powders of the present invention ispreferably not greater than about 1 atomic percent and is morepreferably not greater than about 0.1 atomic percent and even morepreferably is not greater than about 0.01 atomic percent.

[0232] The sulfur-containing phosphor particles of the present inventionare preferably very dense (not porous), as measured by heliumpychnometry. Preferably, the particles have a particle density of atleast about 80 percent of the theoretical density for the host material,more preferably at least about 90 percent of the theoretical density forthe host material and even more preferably at least about 95 percent ofthe theoretical density for the host material.

[0233] The sulfur-containing phosphor particles of the present inventionare also substantially spherical in shape. That is, the particles arenot jagged or irregular in shape. Spherical particles are particularlyadvantageous because they are able to disperse and coat a device, suchas a display panel, more uniformly with a reduced average thickness.Although the particles are substantially spherical, the particles maybecome faceted as the crystallite size increases and approaches theaverage particle size.

[0234] In addition, the sulfur-containing phosphor particles accordingto the present invention advantageously have a low surface area. Theparticles are substantially spherical, which reduces the total surfacearea for a given mass of powder. Further, the elimination of largerparticles from the powder batches eliminates the porosity that isassociated with open pores on the surface of such larger particles. Dueto the elimination of the large particles, the powder advantageously hasa lower surface area. Surface area is typically measured using a BETnitrogen adsorption method which is indicative of the surface area ofthe powder, including the surface area of accessible surface pores onthe surface of the powder. For a given particle size distribution, alower value of a surface area per unit mass of powder indicates solid ornon-porous particles. Decreased surface area reduces the susceptibilityof the phosphor powders to adverse surface reactions, such asdegradation from moisture. This characteristic can advantageously extendthe useful life of the phosphor powders.

[0235] The surfaces of the sulfur-containing phosphor particlesaccording to the present invention are typically smooth and clean with aminimal deposition of contaminants on the particle surface. For example,the outer surfaces are not contaminated with surfactants, as is oftenthe case with particles produced by liquid precipitation routes.

[0236] In addition, the powder batches of sulfur-containing phosphorparticles according to the present invention are substantiallyunagglomerated, that is, they include substantially no hard agglomeratesor particles. Hard agglomerates are physically coalesced lumps of two ormore particles that behave as one large particle. Agglomerates aredisadvantageous in most applications of phosphor powders. It ispreferred that no more than about 1 weight percent of the phosphorparticles in the powder batch of the present invention are in the formof hard agglomerates. More preferably, no more than about 0.5 weightpercent of the particles are in the form of hard agglomerates and evenmore preferably no more than about 0.1 weight percent of the particlesare in the form of hard agglomerates.

[0237] According to one embodiment of the present invention, thesulfur-containing phosphor particles are composite phosphor particles,wherein the individual particles include at least one phosphor phase andat least a second phase associated with the phosphor phase. The secondphase can be a different phosphor compound or can be a non-phosphorcompound, such as a metal oxide. Such composites can advantageouslypermit the use of phosphor compounds in devices that would otherwise beunusable. Further, combinations of different phosphor compounds withinone particle can produce emission of a selected color. For example, onecomposite phosphor particle of the present invention includes a hostmatrix of ZnS:Mn with regions of SrS:Ce dispersed throughout the hostmatrix. The emission of the two phosphor compounds would combine toapproximate white light. Further, in cathodoluminescent applications,the matrix material can accelerate the impingent electrons to enhancethe luminescence.

[0238] According to another embodiment of the present invention, thesulfur-containing phosphor particles are surface modified or coatedphosphor particles that include a particulate coating (FIG. 47d) fornon-particulate (film) coating (FIG. 47a) that substantiallyencapsulates an outer surface of the particles. The coating can be ametal, a non-metallic compound or an organic compound.

[0239] Coatings are often desirable to reduce degradation of thesulfur-containing phosphor compound due to moisture or high densityelectron bombardment in cathodoluminescent devices. For example, metalsulfides such as ZnS are particularly susceptible to degradation due tomoisture and should be completely encapsulated to reduce or eliminatethe degradation reaction. Other phosphors are known to degrade in anelectron beam operating at a high current density, such as in FED's. Thethin, uniform coatings according to the present invention willadvantageously permit use of the phosphor powders under low voltage,high current conditions. Coatings also create a diffusion barrier suchthat activator ions (e.g. Cu and Mn) cannot transfer from one particleto another, thereby altering the luminescence characteristics. Coatingscan also control the surface energy levels of the particles.

[0240] The coating can be a metal, metal oxide or other inorganiccompound such as a metal sulfide, or can be an organic compound. Forexample, a metal oxide coating can advantageously be used, such as ametal oxide selected from the group consisting of SiO₂, MgO, Al₂O₃, ZnO,SnO₂, SnO, ZrO₂, B₂O₅, TiO₂, CuO, Cu₂O, In₂O₃ or In_(x)Sn_(1-x)O₂.Particularly preferred are SiO₂ and Al₂O₃ coatings. Semiconductive oxidecoatings such as SnO₂ or In₂O₃ can be advantageous in some applicationsdue to the ability of the coating to absorb secondary electrons that areemitted by the phosphor. Metal coatings, such as copper, can be usefulfor phosphor particles used in direct current electroluminescentapplications, discussed hereinbelow. In addition, phosphate coatings,such as zirconium phosphate or aluminum phosphate, can also beadvantageous for use in some applications.

[0241] The coatings should be relatively thin and uniform. The coatingshould encapsulate the entire particle; but be sufficiently thin suchthat the coating doesn't interfere with light transmission. Preferably,the coating has an average thickness of not greater than about 200nanometers, more preferably not greater than about 100 nanometers, andeven more preferably not greater than about 50 nanometers. The coatingpreferably completely encapsulates the phosphor particle and thereforeshould have an average thickness of at least about 2 nanometers, morepreferably at least about 5 nanometers. In one embodiment, the coatinghas a thickness of from about 2 to 50 nanometers, such as from about 2to 10 nanometers. Further, the particles can include more than onecoating substantially encapsulating the particles to achieve the desiredproperties.

[0242] The coating, either particulate or non-particulate, can alsoinclude a pigment or other material that alters the lightcharacteristics of the phosphor. Red pigments can include compounds suchas the iron oxides (Fe₂O₃), cadmium sulfide compounds (CdS) or mercurysulfide compounds (HgS). Green or blue pigments include cobalt oxide(CoO), cobalt aluminate (CoAl₂O₄) or zinc oxide (ZnO). Pigment coatingsare capable of absorbing selected wavelengths of light leaving thephosphor, thereby acting as a filter to improve the color contrast andpurity, particularly in CRT devices.

[0243] In addition, the phosphor particles can be coated with an organiccompound such as PMMA (polymethylmethacrylate), polystyrene or similarorganic compounds, including surfactants that aid in the dispersionand/or suspension of the particles in a flowable medium. The organiccoating is preferably not greater than about 100 nanometers thick and issubstantially dense and continuous about particle. The organic coatingscan advantageously prevent corrosion of the phosphor particles,especially in electroluminescent lamps, and also can improve thedispersion characteristics of the particles in a paste or other flowablemedium.

[0244] The coating can also be comprised of one or more monolayercoatings, such as from about 1 to 3 monolayer coatings. A monolayercoating is formed by the reaction of an organic or an inorganic moleculewith the surface of the phosphor particles to form a coating layer thatis essentially one molecular layer thick. In particular, the formationof a monolayer coating by reaction of the surface of the phosphor powderwith a functionalized organo silane such as halo- or amino-silanes, forexample hexamethyldisilazane or trimethylsilylchloride, can be used tomodify and control the hydrophobicity and hydrophilicity of the phosphorpowders. Such coatings allow for greater control over the dispersioncharacteristics of the phosphor powder in a wide variety of pastecompositions and other flowable mediums.

[0245] The monolayer coatings may also be applied to phosphor powdersthat have already been coated with an organic or inorganic coating, thusproviding better control over the corrosion characteristics (through thethicker coating) as well as dispersibility (through the monolayercoating) of the phosphor powder.

[0246] As a direct result of the foregoing powder characteristics, thesulfur-containing phosphor powders of the present invention have manyunique and advantageous properties that are not found in phosphorpowders known heretofore.

[0247] The sulfur-containing phosphor powders of the present inventionhave a high efficiency, sometimes referred to as quantum efficiency.Efficiency is the overall conversion rate of excitation energy(electrons or photons) to visible photons emitted. According to oneembodiment of the present invention, the efficiency of the phosphorpowder is at least about 90%. The near perfect efficiency of thephosphor powders according to the present invention is believed to bedue to the high crystallinity and homogenous distribution of activatorion in the host material.

[0248] The phosphor powders also have well-controlled colorcharacteristics, sometimes referred to as emission spectrumcharacteristics or chromaticity. This important property is due to theability to precisely control the composition of the host material, thehomogenous distribution of the activator ion and the high purity of thepowders. For example, the ability to form mixed metal sulfides ofvarying compositions enables the characteristic wavelength of emissionto be controllably shifted to obtain different colors.

[0249] The phosphor powders also have improved decay time, also referredto as persistence. Persistence is referred to as the amount of time forthe light emission to decay to 10% of its brightness. Phosphors withlong decay times can result in blurred images when the image movesacross the display. The improved decay time of the phosphor powders ofthe present invention is believed to be due to the homogenousdistribution of activator ion in the host material.

[0250] The phosphor powders also have an improved brightness over priorart phosphor powders. That is, under a given application of energy, thephosphor powders of the present invention produce more light.

[0251] Thus, the sulfur-containing phosphor powders of the presentinvention have a unique combination of unique properties that are notfound in conventional phosphor powders. The powders can advantageouslybe used to form a number of intermediate products, for example pastes orslurries, and can be incorporated into a number of devices, wherein thedevices will have significantly improved performance resulting directlyfrom the characteristics of the phosphor powders of the presentinvention. The devices can include light-emitting lamps and displaydevices for visually conveying information and graphics. Such displaydevices include traditional CRT-based display devices, such astelevisions, and also include flat panel displays. Flat panel-displaysare relatively thin devices that present graphics and images without theuse of a traditional picture tube and operate with modest powerrequirements. Generally, flat panel displays include a phosphor powderselectively dispersed on a viewing panel, wherein the excitation sourcelies behind and in close proximity to the panel. Flat panel displaysinclude liquid crystal displays (LCD), plasma display panels (PDP's)electroluminescent (EL) displays, and field emission displays (FED'S).

[0252] CRT devices, utilizing a cathode ray tube, include traditionaldisplay devices such as televisions and computer monitors. CRT's operateby selectively firing electrons from one or more cathode ray tubes atcathodoluminescent phosphor particles which are located in predeterminedregions (pixels) of a display screen. The cathode ray tube is located ata distance from the display screen which increases as screen sizeincreases. By selectively directing the electron beam at certain pixels,a full color display with high resolution can be achieved.

[0253] A CRT display device is illustrated schematically in FIG. 50. Thedevice 1002 includes 3 cathode ray tubes 1004, 1006 and 1008 located inthe rear portion of the device. The cathode ray tubes generateelectrons, such as electron 1010. An applied voltage of 20 to 30 kVaccelerates the electrons toward the display screen 1012. In a colorCRT, the display screen is patterned with red (R), green (G) and blue(B) phosphors, as is illustrated in FIG. 51. Three colored phosphorpixels are-grouped in close proximity, such as group 1014, to producemulticolor images. Graphic output is created be selectively directingthe electrons at the pixels on the display screen 1012 using, forexample, electromagnets 1016. The electron beams are rastered in a leftto right, top to bottom fashion to create a moving image. The electronscan also be filtered through an apertured metal mask to block electronsthat are directed at the wrong phosphor.

[0254] The phosphor powder is typically applied to the CRT displayscreen using a slurry. The slurry is formed by suspending the phosphorparticles in an aqueous solution which can also include additives suchas PVA (polyvinyl alcohol) and other organic compounds to aid in thedispersion of the particles in the solution as well as other compoundssuch as metal chromates. The display screen is placed in a coatingmachine, such as a spin coater, and the slurry is deposited onto theinner surface of the display screen and spread over the entire surface.The display screen is spun to thoroughly coat the surface and spin awayany excess slurry. The slurry on the screen is then dried and exposedthrough a shadow mask having a predetermined dot-like or stripe-likepattern. The exposed film is developed and excess phosphor particles arewashed away to form a phosphor screen having a predetermined pixelpattern. The process can be performed in sequence for different colorphosphors to enable a full color display to be produced.

[0255] It is generally desired that the pixels are formed with a highlyuniform phosphor powder layer thickness. The phosphors should not peelfrom the display screen and no cross contamination of the coloredphosphors should occur. These characteristics are significantlyinfluenced by the morphology, size and surface condition of the phosphorparticles.

[0256] CRT devices typically employ phosphor particles rather thanthin-film phosphors due to the high luminescence requirements. Theresolution of images on powdered phosphor screens can be improved if thescreen is made with particles having a small size and uniform sizedistribution, such as the phosphor particles according to the presentinvention. Image quality on the CRT device is also influenced by thepacking voids of the particles and the number of layers of phosphorparticles which are not involved in the generation ofcathodoluminescence. That is, particles which are not excited by theelectron beam will only inhibit the transmission of luminescence throughthe device. Large particles and aggregated particles both form voids andfurther contribute to loss of light transmission. Significant amounts oflight can be scattered by reflection in voids. Further, for a highquality image, the phosphor layer should have a thin and highly uniformthickness. Ideally, the average thickness of the phosphor layer shouldbe about 1.5 times the average particle size of the phosphor particles.

[0257] CRT's typically operate at high voltages such as from about 20 kVto 30 kV. Phosphors used for CRT's should have high brightness and goodchromaticity. Sulfur-containing phosphors which are particularly usefulin CRT devices include ZnS:Cu or Al for green, ZnS:Ag, Au or Cl for blueand Y₂O₂S:Eu for red. Other sulfur-containing phosphors, such as CdS:Ag,Au or Cl, can be used in CRT devices as well. Mixed metal sulfides suchas Zn_(x)Cd_(1-x)S:Ag or Cu can also be advantageous. The phosphorparticles can advantageously be coated in accordance with the presentinvention to prevent degradation of the host material or diffusion ofactivator ions. Silica or silicate coatings can also improve therheological properties of the phosphor slurry. The particles can alsoinclude a pigment coating, such as particulate Fe₂O₃, to modify andenhance the properties of the emitted light.

[0258] The introduction of high-definition televisions (HDTV) hasincreased the interest in projection television (PTV). In this concept,the light produced by three independent cathode ray tubes is projectedonto a faceplate on the tube that includes particulate phosphors, toform 3 colored projection images. The three images are projected onto adisplay screen by reflection to produce a full color image. Because ofthe large magnification used in imaging, the phosphors on the faceplateof the cathode ray tube must be excited with an intense and smallelectron spot. Maximum excitation density may be two orders of magnitudelarger than with conventional cathode ray tubes. Typically, theefficiency of the phosphor decreases with increasing excitation density.For the foregoing reasons, the powders of the present invention would beparticularly useful in HDTV applications.

[0259] One of the problems with CRT-based devices is that they are largeand bulky and have significant depth as compared to the screen size.Therefore, there is significant interest in developing flat paneldisplays to replace CRT-based devices in many applications.

[0260] Flat panel displays (FPD's) offer many advantages over CRT'sincluding lighter weight, portability and decreased power requirements.Flat panel displays can be either monochrome or color displays. It isbelieved that flat panel displays will eventually replace the bulky CRTdevices, such as televisions, with a thin product that can be hung on awall, like a picture. Currently, flat panel displays can be madethinner, lighter and with lower power consumption than CRT devices, butnot with the visual quality and cost performance of a CRT device.

[0261] The high electron voltages and small currents traditionallyrequired to activate phosphors efficiently in a CRT device have hinderedthe development of flat panel displays. Phosphors for flat paneldisplays such as field emission displays must typically operate at alower voltage, higher current density and higher efficiency thanphosphors used in existing CRT devices. The low voltages used in suchdisplays result in an electron penetration depth in the range of severalmicrometers down to tens of nanometers, depending on the appliedvoltage. Thus, the control of the size and crystallinity of the phosphorparticles is critical to device performance. If large or agglomeratedpowders are used, only a small fraction of the electrons will interactwith the phosphor. Use of phosphor powders having a wide sizedistribution can also lead to non-uniform pixels and sub-pixels, whichwill produce a blurred image.

[0262] One type of FPD is a plasma display panel (PDP). Plasma displayshave image quality that is comparable to current CRT devices and can beeasily scaled to large sizes such as 20 to 60 diagonal inches. Thedisplays are bright and lightweight and have a thickness of from about1.5 to 3 inches. A plasma display functions in a similar manner asfluorescent lighting. In a plasma display, a plasma source, typically agas mixture, is placed between an opposed array of addressableelectrodes and a high energy electric field is generated between theelectrodes. Upon reaching a critical voltage, a plasma is formed fromthe gas and UV photons are emitted by the plasma. Color plasma displayscontain three-color photoluminescent phosphor particles deposited on theinside of the glass faceplate. The phosphors selectively emit light whenilluminated by the photons. Plasma displays operate at relatively lowcurrents and can be driven either by an AC or DC signal. AC plasmasystems use a dielectric layer over the electrode, which forms acapacitor. This impedance limits current and provides a necessary chargein the gas mixture.

[0263] A cross-section of a plasma display device is illustrated in FIG.52. The plasma display 1040 comprises two opposed panels 1042 and 1044in parallel opposed relation. A working gas is disposed and sealedbetween the two opposing panels 1042 and 1044. The rear panel 1044includes a backing plate 1046 on which are printed a plurality ofelectrodes 1048 (cathodes) which are in parallel spaced relation. Aninsulator 1050 covers the electrodes and spacers 1052 are utilized toseparate the rear panel 1044 from the front panel 1042.

[0264] The front panel 1042 includes a glass face plate 1054 which istransparent when observed by the viewer (V). Printed onto the rearsurface of the glass face plate 1054 are a plurality of electrodes 1056(anodes) in parallel spaced relation. An insulator 1058 separates theelectrode from the pixels of phosphor powder 1060. The phosphor powder1060 is typically applied using a thick film paste. When the display1040 is assembled, the electrodes 1048 and 1056 are perpendicular toeach other, forming an XY grid. Thus, each pixel of phosphor powder canbe activated by the addressing an XY coordinate defined by theintersecting electrodes 1048 and 1056.

[0265] One of the problems currently encountered in plasma displaydevices is the long decay time of the phosphor particles, which createsa “tail” on a moving image. Through control of the phosphor chemistry,such decay-related problems can be reduced. Further, the spherical,non-agglomerated nature of the phosphor particles improves theresolution of the plasma display panel.

[0266] One sulfur-containing phosphor that is particularly useful inplasma displays is Gd₂O₂S:Tb for green. Preferably, such a phosphor iscoated with a uniform coating having a thickness of from about 2 to 10nanometers.

[0267] Another type of flat panel display is a field emission display(FED). These devices advantageously eliminate the size, weight and powerconsumption problems of CRT's while maintaining comparable imagequality, and therefore are particularly useful for portable electronics,such as for laptop computers. FED's generate electrons from millions ofcold microtip emitters with low power emission that are arranged in amatrix addressed array with several thousand tip emitters allocated toeach pixel in the display. The microtip emitters are typically locatedapproximately 0.2 millimeter from a cathodoluminescent phosphor screen,which generates the display image. This allows for a thin, light-weightdisplay.

[0268]FIG. 53 illustrates a high-magnification, schematic cross-sectionof an FED device according to an embodiment of the present invention.The FED device 1080 includes a plurality of microtip emitters 1082mounted on a cathode 1084 which is attached to a backing plate 1086. Thecathode is separated from a gate or emitter grid 1088 by an insulatingspacer 1090. Opposed to the cathode 1084 and separated by a vacuum is afaceplate assembly 1091 including phosphor pixel 1092 and a transparentanode 1094. The phosphor pixel layers can be deposited using a paste orelectrophoretically. The FED can also include a transparent glasssubstrate 1096 onto which the anode 1094 is printed. During operation, apositive voltage is applied to the emitter grid 1088 creating a strongelectric field at the emitter tip 1082. The electrons 1098 migrate tothe faceplate 1091 which is maintained at a higher positive voltage. Thefaceplate collector bias is typically about 1000 volts. Several thousandmicrotip emitters 1082 can be utilized for each pixel in the display.

[0269] Sulfur-containing phosphors which are particularly useful for FEDdevices include thiogallates such as SrGa₂S₄:Eu for green, SrGa₂S₄:Cefor blue, ZnS:Ag or Cl for blue and SrS:Ga or Cu for blue. ZnS:Ag or Cucan also be used for higher voltage FED devices. For use in FED devices,these phosphors are preferably coated, such as with a very thin metaloxide coating, since the high electron beam current densities can causebreakdown and dissociation of the sulfur-containing phosphor hostmaterial. Dielectric coatings such as SiO₂ and Al₂O₃ can be used.Further, semiconducting coatings such as SnO or In₂O₃ can beparticularly advantageous to absorb secondary electrons.

[0270] Coatings for the sulfur-containing FED phosphors preferably havean average thickness of from about 1 to 10 nanometers, more preferablyfrom about 1 to 5 nanometers. Coatings having a thickness in excess ofabout 10 nanometers will decrease the brightness of the device since theelectron penetration depth of 1-2 kV electrons is only about 10nanometers. Such thin coatings can advantageously be monolayer coatings,as is discussed above.

[0271] The primary obstacle to further development of FED's is the lackof adequate phosphor powders. FED's require low-voltage phosphormaterials, that is, phosphors which emit sufficient light under lowapplied voltages, such as less than about 500 volts, and high currentdensities. The sulfur-containing phosphor powders of the presentinvention advantageously have improved brightness under such low appliedvoltages and the coated phosphor particles resist degradation under highcurrent densities. The improved brightness can be attributed to the highcrystallinity and high purity of the particles. Phosphor particles withlow crystallinity and high impurities due to processes such as millingdo not have the desirable high brightness. The phosphor particles of thepresent invention also have the ability to maintain the brightness andchromaticity over long periods of time, such as in excess of 10,000hours. Further, the spherical morphology of the phosphor powder improveslight scattering and therefore improves the visual properties of thedisplay. The small average size of the particles is advantageous sincethe electron penetration depth is only several nanometers, due to thelow applied voltage.

[0272] For each of the foregoing display devices, cathode ray tubedevices and flat panel display devices including plasma display panelsand field emission devices, it is important for the phosphor layer to beas thin and uniform as possible with a minimal number of voids. FIG. 54schematically illustrates a lay down of large agglomerated particles ina pixel utilizing conventional phosphor powders. The device 1100includes a transparent viewing screen 1102 and, in the case of an FED, atransparent electrode layer 1104. The phosphor particles 1106 aredispersed in pixels 1108. The phosphor particles are large andagglomerated and result in a number of voids and unevenness in thesurface. This results in decreased brightness and decreased imagequality.

[0273]FIG. 55 illustrates the same device fabricated utilizing powdersaccording to the present invention. The device 1110 includes transparentviewing screen 1112 and a transparent electrode 1114. The phosphorpowders 1116 are dispersed in pixels in 1118. The pixels are thinner andmore uniform than the conventional pixel. In a preferred embodiment, thephosphor layer constituting the pixel has an average thickness of notgreater than about 3 times the average particle size of the powder,preferably not greater than about 2 times the average particle size andeven more preferably not greater than about 1.5 times the averageparticle size. This unique characteristic is possible due to the uniquecombination of small particle size, narrow size distribution andspherical morphology of the phosphor particles. The device willtherefore produce an image having much higher resolution due to theability to form smaller, more uniform pixels and much higher brightnesssince light scattering is significantly reduced and the amount of lightlost due to non-luminescent particles is reduced.

[0274] Electroluminescent displays (EL displays) work byelectroluminescence. EL displays are very thin structures which can havevery small screen sizes, such as few inches diagonally, while producinga very high resolution image. These displays, due to the very smallsize, are utilized in many military applications where size is a strictrequirement such as in aircraft cockpits, small hand-held displays andheads-up displays. These displays function by applying a high electricpotential between two addressing electrodes. EL displays are mostcommonly driven by an A.C. electrical signal. The electrodes are incontact with a semiconducting phosphor thin-film and the largepotential: difference creates hot electrons which move through thephosphor, allowing for excitation followed by light emission.

[0275] An EL display is schematically illustrated in FIGS. 56 and 57.The EL display device 1120 includes a phosphor layer 1122 sandwichedbetween two dielectric insulating layers 1124 and 1126. On the back sideof the insulating layers is a backplate 1128 which includes rowelectrodes 1130. On the front of the device is a glass faceplate 1132which includes transparent column electrodes 1134, such as electrodesmade from transparent indium tin oxide.

[0276] While current electroluminescent display configurations utilize athin film phosphor layer 1122 and do not typically utilize phosphorpowders, the use of very small monodispersed phosphor particlesaccording to the present invention is advantageous for use in suchdevices. For example, small monodispersed particles could be depositedon a glass substrate using a thick film paste and sintered to produce awell connected film and therefore could replace the expensive andmaterial-limited CVD technology currently used to deposit such films.Such a well-connected film could not be formed from large, agglomeratedphosphor particles. Similarly, composite phosphor particles are a viablealternative to the relatively expensive multilayer stack currentlyemployed in electroluminescent displays. Thus, a composite phosphorparticle comprising the phosphor and a dielectric material could beused.

[0277] Particularly preferred phosphors for use in electroluminescentdisplay applications include the metal sulfides such as ZnS:Cu, BaS:Ce,CaS:Ce, SrS:RE (RE=rare earth), and ZnS:Mn. Further, mixed metalsulfides such as Sr_(x)Ca_(y)Ba_(1-x-y)S_(N):Ce can be used. Further,the thiogallate phosphors according to the present invention can alsohave advantages for use in electroluminescent displays.

[0278] Another display device for which the phosphors according to thepresent invention are useful are liquid crystal displays (LCD), and inparticular active matrix liquid crystal displays (AMLCD). Such LCDdisplays are currently used for a majority of laptop computer displayscreens. The key element of an LCD device is the liquid crystal materialwhich can be influenced by an electric field to either transmit light orblock light.

[0279] LCD displays work by producing a light field and filtering lightfrom the field using the liquid crystal material to produce an image. Asa result, only about 3% of the light emitted by the underlying phosphorscreen is transmitted to the viewer. Therefore, the phosphors accordingto the present invention having a higher brightness can provide LCDdisplays having increased brightness and contrast.

[0280] Another use for phosphor powders according to the presentinvention is in the area of electroluminescent lamps. Electroluminescentlamps are formed on a rigid or flexible substrate, such as a polymersubstrate, and are commonly used as back lights for membrane switches,cellular phones, watches, personal digital assistants and the like. Asimple electroluminescent lamp is schematically illustrated in FIG. 58.The device 1140 includes a phosphor powder/polymer composite 1142 issandwiched between two electrodes 1144 and 1146, the front electrode1144 being transparent. The composite layer 1142 includes phosphorparticles 1148 dispersed in a polymer matrix 1150.

[0281] Electroluminescent lamps can also be formed on rigid substrates,such as stainless steel, for use in highway signage and similar devices.The rigid device includes a phosphor particle layer, a ceramicdielectric layer and a transparent conducting electrode layer. Suchdevices are sometimes referred to as solid state ceramicelectroluminescent lamps (SSCEL). To form such rigid devices, a phosphorpowder is typically sprayed onto a rigid substrate.

[0282] Electroluminescent lamp manufacturers currently have only simplemetal sulfides such as ZnS phosphor powder host material at theirdisposal. ZnS:Cu produces a blue color, while ZnS:Mn, Cu produces anorange color. These materials have poor reliability and brightness,especially when filtered to generate other colors. Additional colors,higher reliability and higher brightness powders are critical needs forthe electroluminescent lamp industry to supply designers with theability to penetrate new market segments. The phosphor layers shouldalso be thinner and denser, without sacrificing brightness, to minimizewater intrusion and eliminate light scattering. Higher brightnesselectroluminescent lamps require thinner phosphor layers, which requiressmaller particle size phosphor powders that cannot produced byconventional methods. Such thinner layers will also use less phosphorpowder. Presently available EL lamps utilize powders having an averagesize of about 5 μm or higher. The phosphor powders of the presentinvention having a small particle size and narrow size distribution,will enable the production of brighter and more reliable EL lamps thathave an increased life-expectancy. Further, the phosphor powders of thepresent invention will enable the production of EL lamps wherein thephosphor layer has a significantly reduced thickness, withoutsacrificing brightness or other desirable properties. Conventional ELlamps have phosphor layers on the order of 100 μm thick. The powders ofthe present invention advantageously enable the production of an EL lamphaving a phosphor layer that is not greater than about 15 μm thick, suchas not greater than about 10 μm thick. The phosphor layer is preferablynot thicker than about 3 times the weight average particle size, morepreferably not greater than about 2 times the weight average particlesize.

[0283] As discussed above, preferred electroluminescentsulfur-containing phosphors for use in electroluminescent lamps includeZnS:Cu for blue or blue-green and ZnS:Mn, Cu for orange. Other materialsthat are desirable for EL lamp applications include BaS:RE, Cu or Mn,CaS:RE or Mn, SrS:RE or Mn, and Sr_(x)Ca_(y)Ba_(1-x-y)S:RE (where RE isa rare earth element) for other colors. CaS:Ga or Cu and SrS:Ga or Cuare also useful. The thiogallate phosphors of the present invention,such as SrGa₂S₄ and CaGa₂S₄, can be particularly advantageous for use inelectroluminescent lamps. As is discussed above, many of these phosphorscannot be produced using conventional techniques and therefore have notbeen utilized in EL lamp applications. When used in an EL lamp, thesephosphors should be coated to prevent degradation of the phosphor due tohydrolysis or other adverse reactions. Preferably, such a coating has anaverage thickness of from about 2 to 50 nanometers.

[0284] As stated above, electroluminescent lamps are becomingincreasingly important for back lighting alphanumeric displays in smallelectronic devices such as cellular phones, pagers, personal digitalassistants, wrist watches, calculators and the like. They are alsouseful in applications such as instrument panels, portable advertisingdisplays, safety lighting, emergency lighting for rescue and safetydevices, photographic backlighting, membrane switches and other similarapplications. One of the problems associated with electroluminescentdevices is that they generally require the application of alternatingcurrent (AC) voltage to produce light. A significant obstacle to thedevelopment of the useful direct current (DC) electroluminescent (DCEL)devices is a need for a phosphor powder that will function adequatelyunder a DC electric field. The phosphor powder for functioning under aDC electric field should meet at least three requirements: 1) theparticles should have a small average particle size; 2) the particlesshould have a uniform size, that is, the particles should have a narrowsize distribution with no large particles or agglomerates; and 3) theparticles should have good luminescence properties, particularly a highbrightness. The phosphor powders of the present invention advantageouslymeet these requirements. Therefore, the phosphor powders of the presentinvention will advantageously permit the use of electroluminescentdevices without requiring an inverter to convert a DC voltage to an ACvoltage. Such devices are riot believed to be commercially available atthis time. When utilized in a device applying DC voltage, it ispreferred to coat the phosphor particles with a thin layer of aconductive compound, such as a metal, for example copper metal, or aconductive compound such as copper sulfide.

[0285] The sulfur-containing phosphors of the present invention are alsouseful for security purposes. In this application, phosphors which areundetectable under normal lighting, become visible upon illumination bya particular energy, typically ultraviolet or infrared radiation,emitting characteristic wavelengths, typically in the ultravioletspectrum.

[0286] For security purposes, the phosphor particles are dispersed intoa liquid vehicle which can be applied onto a surface by standard inkdeposition methods, such as by using an ink jet or a syringe, or byscreen printing. The phosphor particles of the present invention, havinga small size and narrow size distribution, will permit better controlover the printed feature size and complexity. The methodology of thepresent invention also permits unique combinations of phosphor compoundsthat are not available using conventional methods. Such taggents can beapplied to currency, secure documentation, explosives and munitions, orany other item that may require coding.

[0287] Useful phosphor compounds for taggent applications include metalsulfides doped with at least two activator ions, such as SrS:Sm, Ce orCaS:Sm, Eu. Oxysulfides, such as Y₂O₂S:Er, Yb are also useful. Suchphosphors emit visible light upon excitation by an infrared source. Thesulfur-containing phosphor powders of the present invention provide manyadvantages in such applications. The small, monodispersed nature of theparticles makes the particles easy to supply in smaller quantities.Further, different-colors for specific security purposes can be achievedby using mixed metal sulfides wherein the ratio of the sulfides isvaried to obtain different wavelengths of color, as is discussed above.

[0288] Up-convertor phosphors are also useful in immunoassayapplications. Immunoassays are bioactive agent detectors designed todetect chemicals in the bloodstream, such as sugars, insulin ornarcotics. The phosphor is delivered to the biological substrate and theinteraction between the substrate and the underlying phosphor results ina detected color shift which can be correlated with the concentration ofthe initial bioactive molecule present in the sample. For example,incident infrared light can result in a detectable ultraviolet signalfrom the phosphor. The up-convertor phosphors of the present inventionused for such immunoassay applications preferably have an averageparticle size of from about 0.1 μm to about 0.4 μm and are preferablycoated to bind the biologically active molecule. Preferredsulfur-containing phosphors include SrS:Sm, Eu as well as oxysulfides.The particles are frequently coated, such as with SiO₂, to enhance tobinding of the phosphor to the biological substrate and forbiocompatibility.

[0289] In addition to the foregoing, the sulfur-containing phosphors ofthe present invention can also be used as target materials for thedeposition of phosphor thin-films by electron beam deposition,sputtering and the like. The particles can be consolidated to form thetarget for the process. The homogenous concentration of activator ionsin the particles will lead to more uniform and brighter film. Thephosphor powders can also be used to adjust the color of light emittingdiodes.

[0290] For many of the foregoing applications, phosphor powders areoften dispersed within a paste, or ink, which is then applied to asurface to obtain a phosphorescent layer. These pastes are commonly usedfor electroluminescent lamps, FED's, plasma displays, CRT's, lampphosphors and thick-film electroluminescent displays. The powders of thepresent invention offer many advantages when dispersed in such a paste.For example, the powders will disperse better than non-spherical powdersof wide size distribution and can therefore produce thinner and moreuniform layers with a reduced lump-count. Such a thick film paste willproduce a brighter display. The packing density of the phosphors willalso be higher. The number of processing steps can also beadvantageously reduced. For example, in the preparation ofelectroluminescent lamps, two dielectric layers are often needed tocover the phosphor paste layer because many of the phosphor particleswill be large enough to protrude through one layer. Spherical particlesthat are substantially uniform in size will eliminate this problem andthe EL lamp will advantageously require one dielectric layer.

[0291] One preferred class of intermediate products according to thepresent invention are thick film paste compositions, also referred to asthick film inks. These pastes are particularly useful for theapplication of the phosphor particles onto a substrate, such as for usein a flat panel display, as is discussed more fully hereinabove.

[0292] In the thick film process, a viscous paste that includes afunctional particulate phase, such as phosphor powder, is screen printedonto a substrate. A porous screen fabricated from stainless steel,polyester, nylon or similar inert material is stretched and attached toa rigid frame. A predetermined pattern is formed on the screencorresponding to the pattern to be printed. For example, a UV sensitiveemulsion can be applied to the screen and exposed through a positive ornegative image of the design pattern. The screen is then developed toremove portions of the emulsion in the pattern regions.

[0293] The screen is then affixed to a printing device and the thickfilm paste is deposited on top of the screen. The substrate to beprinted is then positioned beneath the screen and the paste is forcedthrough the screen and onto the substrate by a squeegee that traversesthe screen. Thus, a pattern of traces and/or pads of the paste materialis transferred to the substrate. The substrate with the paste applied ina predetermined pattern is then subjected to a drying and heatingtreatment to adhere the functional phase to the substrate. For increasedline definition, the applied paste can be further treated, such asthrough a photolithographic process, to develop and remove unwantedmaterial from the substrate.

[0294] Thick film pastes have a complex chemistry and generally includea functional phase, a binder phase and an organic vehicle phase. Thefunctional phase can include the phosphor powders of the presentinvention which provide a luminescent layer on a substrate. The particlesize, size distribution, surface chemistry and particle shape of theparticles all influence the rheology of the paste.

[0295] The binder phase is typically a mixture of inorganic binders suchas metal oxide or glass frit powders. For example, PbO based glasses arecommonly used as binders. The function of the binder phase is to controlthe sintering of the film and assist the adhesion of the functionalphase to the substrate and/or assist in the sintering of the functionalphase. Reactive compounds can also be included in the paste to promoteadherence of the functional phase to the substrate.

[0296] Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins or other organics whose primaryfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetate, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters.

[0297] The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste. Thepowder is often dispersed in the paste and then repeatedly passedthrough a roll-mill to mix the paste. The roll mill can advantageouslybreak-up soft agglomerates of powders in the paste. Typically, the thickfilm paste will include from about 5 to about 95 weight percent, such asfrom about 60 to 85 weight percent, of the functional phase, includingthe phosphor powders of the present invention.

[0298] Some applications of thick film pastes, such as for forminghigh-resolution display panels, require higher tolerances than can beachieved using standard thick-film technology, as is described above. Asa result, some thick film pastes have photo-imaging capability to enablethe formation of lines and traces with decreased width and pitch. Inthis type of process, a photoactive thick film paste is applied to asubstrate substantially as is described above. The paste can include,for example, a liquid vehicle such as polyvinyl alcohol, that is notcross-linked. The paste is then dried and exposed to ultraviolet lightthrough a photomask to polymerize the exposed portions of paste and thepaste is developed to remove unwanted portions of the paste. Thistechnology permits higher density lines and pixels to be formed. Thecombination of the foregoing technology with the phosphor powders of thepresent invention permits the fabrication of devices with resolution andtolerances as compared to conventional technologies using conventionalphosphor powders.

[0299] Phosphor paste compositions are disclosed in U.S. Pat. No.4,724,161, U.S. Pat. No. 4,806,389, U.S. Pat. No. 4,902,567 which areincorporated herein by reference in their entirety.

EXAMPLES

[0300] In order to demonstrate the advantages of the present invention,the following examples were prepared.

[0301] 1. Simple Metal Sulfides (SrS:Mn)

[0302] 1 gram of strontium carbonate (SrCO₃) was added to 20 ml ofdeionized water. The suspension was stirred and about 1 ml of thioaceticacid (HS(O)CR) and 0.003 grams MnC₂ were added. The strontium carbonaterapidly dissolved to form a clear, pale-yellow solution.

[0303] The solution was placed into contact with an ultrasonic nebulizeroperating at a frequency of about 1.6 MHZ to produce an aerosol ofsolution droplets. A nitrogen carrier gas was used to carry the dropletsinto an elongate tubular furnace heated to a temperature of 600-1500° C.The resulting powder was a substantially phase-pure SrS with Mn²⁺incorporated as an activator ion, a green phosphor. The average particlesize was about 1.0 μm. X-ray diffraction indicated that the particlesconsisted of phase pure SrS with high crystallinity.

[0304] 2. Simple Metal Sulfides (ZnS:Mn)

[0305] Zinc nitrate were placed into a solution with about twoequivalents of thiourea to yield a total of about 3.3 weight percentzinc in the solution. 0.5 mole percent manganese was added to thesolution in the form of (manganese chloride (MnCl₂). The solution wasstirred to yield a fine yellow precipitate.

[0306] The solution was formed into an aerosol as in Example 1 and wascarried in nitrogen gas to an elongate tube furnace heated to a peaktemperature of 950° C. Aerosol droplets having a size of larger thanabout 10 μm were removed from the aerosol using an impactor beforeentering the furnace. The resulting powder had an average particle sizeof about 0.75 μm and included substantially no particles having a sizegreater than about 1.1 μm. As is illustrated by FIG. 59, the particleshad a substantially spherical morphology and a small particle size.

[0307] Other simple metal sulfide phosphors that were produced inaccordance with the present invention in a similar manner to Examples 1and 2 were CaS, MgS and BaS incorporating various activator ions. An SEMphotomicrograph of a BaS:Ce powder produced according to the presentinvention is illustrated in FIG. 60.

[0308] 3. Mixed Metal Sulfides (Ca_(0.5)Sr_(0.5)S:Ce)

[0309] An aqueous solution was prepared by mixing calcium and strontiumcarbonates with thioacetic acid for a mole ratio of calcium to strontiumof about 1:1. About 0.5 mole percent cerium was added in the form ofcerium nitrate Ce(NO₃)₃. The solution was a atomized as in Example 1 andwas carried using nitrogen gas into ah elongate tube furnace heated to apeak temperature of 1100° C. The resulting particles had a small averageparticle size and had a spherical morphology, similar to the powderillustrated in FIG. 59.

[0310] Thus, mixed metal sulfide phosphors can be produced in accordancewith the present invention. Other examples of mixed metal sulfides whichwere produced in accordance with this example include Ca_(x)Sr_(1-x)Sand Mg_(x)Sr_(1-x)S.

[0311] 4. ZnS:M (Colloid Route)

[0312] Two equivalents of thioacetic acid were added to basic zinccarbonate (Zn_(x)(OH)_(y)(CO₃)₂) and about 0.5 mole percent of a metaldopant was added in the form of a metal salt. After about 30 minutes,the clear solution precipitates a fine yellow powder of ZnS. The finepowder is colloidal in form and had an average particle size of lessthan about 0.5 micrometers. The solution was atomized to form dropletsand was carried into a furnace at 950° C. using nitrogen gas.

[0313] The particles had an increased crystallinity as opposed toparticles formed from soluble precursors (Example 1). The powder isillustrated in FIG. 61. The increased crystallinity will produce higherbrightness in a device such as an FED or electroluminescent lamp.

[0314] 5. Coated ZnS

[0315] Zinc sulfide phosphor powder was coated according to the presentinvention by contacting coating precursors with the particles at anelevated temperature. Selected coating compounds were alumina, tin oxideand silica. The precursors were Al (secBuO)₃ for alumina; (Me)₂ SnCl₂and (nBu)₂Sn(OAc)₂ for tin oxide (SnO₂) and tetraethylorthosilicate(TEOS) for silica. The precursors were introduced into the furnace andcontacted with the ZnS particles at a reaction temperature of 500° C. to700° C. in nitrogen. The ZnS powder was fed into the furnace using aWright dust feeder and coatings were deposited by reaction in the gasphase to form a coating by a GPC mechanism. The coating improvesresistance to degradation from water and other influences during use ofthe phosphor powders.

[0316] 6. Annealing of Phosphor Powders

[0317] Various phosphor produced in accordance with the presentinvention were annealed under varying conditions to determine the effectof the annealing treatment.

[0318] An SrS:Eu phosphor which was formed at 1000° C. and included 1atomic weight percent europium was annealed in static argon for 1 minuteat temperatures varying from 700° C. to 1100° C. It was observed thatthe maximum average crystallite size of about 52 nanometers was obtainedat about 800° C. A corresponding peak in the photoluminescent intensitywas observed corresponding to this annealing temperature.

[0319] An SrS:Eu phosphor which was processed at 1100° C. and included0.25 atomic percent europium was annealed in static argon for 1 hour atvarious temperatures. A peak in the photoluminescent intensity wasobserved when the powder was annealed at a temperature of about 950° C.When the same phosphor was annealed in flowing argon for about 30minutes, a similar peak was observed at about 900° C. Thus, it wasconcluded that the optimum annealing temperature for this metal sulfidewas about 800 to 900° C.

[0320] 7. Thiogallate Compounds

[0321] As is discussed herein, thiogallate compounds are preferablyproduced using a spray-conversion process. Such a process is required toproduce a substantially phase pure thiogallate compound with lowimpurities and a desirable morphology.

[0322] An aqueous solution was formed including 2 mole equivalentsgallium nitrate (Ga(NO₃)₃) and 1 mole equivalent strontium nitrate(Sr(NO₃)₂). About 0.05 mole equivalents of europium nitrate (Eu(NO₃)₃)was also added.

[0323] The aqueous solution was formed into an aerosol, substantially inaccordance with Example 1. The aerosol was carried in air through afurnace heated to a temperature of about 800° C. to spray-convert thesolution. The intermediate product was a SrGa₂O₄ oxide having a smallaverage particle size and low impurities.

[0324] The oxide intermediate product was then roasted at 900° C. undera flowing gas that included H₂S and nitrogen in a 1:1 ratio, for about20 minutes. The resulting powder was substantially phase pure SrGa₂S₄:Eu(3 atomic percent Eu) having good crystallinity and good luminescentcharacteristics.

[0325] While various embodiments of the present invention have beendescribed in detail, it is apparent that modifications and adaptationsof those embodiments will occur to those skilled in the art. However, itis to be expressly understood that such modifications and adaptationsare within the spirit and scope of the present invention.

1-118. (canceled)
 119. A method for the production of asulfur-containing phosphor powder, comprising the steps of: a) formingan aqueous-based solution comprising soluble precursors of asulfur-containing phosphor; b) generating an aerosol of droplets fromsaid aqueous-based solution; c) heating said droplets to form aparticulate intermediate compound that is capable of being post-treatedto form said sulfur-containing phosphor compound; and d) treating saidparticulate intermediate compound to form said sulfur-containingphosphor powder.
 120. A method as recited in claim 119, wherein saidmethod further comprises the step of milling said phosphor powder. 121.A method as recited in claim 119, wherein said method further comprisesthe step of annealing said phosphor powder.
 122. A method as recited inclaim 119, wherein said particulate intermediate compound has an averageparticle size of from about 0.3 to about 3 μm.
 123. A method as recitedin claim 119, wherein said method further comprises the step ofannealing said phosphor powder in contact with sulfur or asulfur-containing compound.
 124. A method as recited in claim 119,wherein said method further comprises the step of annealing saidphosphor powder in contact with H₂S gas at a temperature and for a timesufficient to form said sulfur-containing phosphor powder.
 125. A methodas recited in claim 119, wherein said sulfur-containing phosphor isselected from the Group 2 and Group 12 metal sulfides.
 126. A method asrecited in claim 119, wherein said sulfur-containing phosphor is athiogallate.
 127. A method as recited in claim 119, wherein saidaqueous-based solution further comprises a precursor to an activatorion. 128-239. (canceled)